.NET Docker Containerization Skill

Generate optimized Docker configurations for .NET projects using advanced build techniques, progressive layer publishing, and production-ready multi-stage builds.

What This Skill Does

I will analyze your .NET solution and generate:

1. Optimized Dockerfile with BuildKit features and layer caching
2. Build scripts (Bash/PowerShell) with version tagging
3. .dockerignore file with comprehensive patterns
4. Validation checklist and troubleshooting guidance

Advanced Techniques Applied

BuildKit Frontend (syntax=docker/dockerfile:1-labs)

I use the experimental BuildKit frontend for:

• --parents flag support (preserves directory structure)
• Better caching mechanisms
• Advanced COPY operations
• Improved build performance

Progressive Layer Publishing

For complex projects, I publish in dependency order:

1. Domain layer → Publish first (most stable)
2. Infrastructure/EF Core → Publish second
3. Application/HttpApi → Publish third
4. API Host → Publish last (changes most)

Why? This creates separate layers in /app/publish, optimizing Docker layer caching. When you change only the API code, earlier layers remain cached.

Non-Alpine SDK with Alpine Runtime

• Build stage: Uses full SDK (not Alpine) for better compatibility
• Runtime stage: Uses Alpine for minimal footprint
• Benefit: Avoid Alpine SDK build issues while keeping final image small

Project Analysis

Detection Process

I'll examine:

• Solution file (*.sln) location and structure
• All project files (*.csproj) and their dependencies
• Main entry point (typically *.Host, *.Api, *.HttpApi.Host)
• .NET version from <TargetFramework> tags
• Existence of common.props (ABP Framework indicator)
• Project architecture (Simple, DDD, ABP, Clean Architecture)

Dependency Graph Mapping

I'll build a dependency graph to determine:

• Which projects reference which
• Optimal layer ordering for caching
• Whether progressive publishing is beneficial

Simple projects (≤3): Single publish step
Complex projects (≥4): Progressive multi-layer publishing

Dockerfile Generation

Standard Template Structure

# syntax=docker/dockerfile:1-labs
# BuildKit frontend for advanced features (--parents flag)

# Runtime base: Alpine for minimal size
FROM mcr.microsoft.com/dotnet/aspnet:{VERSION}-alpine AS base
USER app
WORKDIR /app
EXPOSE 8080
EXPOSE 8081

# Build stage: Full SDK (not Alpine) for compatibility
FROM mcr.microsoft.com/dotnet/sdk:{VERSION}-alpine AS publish
ARG BUILD_CONFIGURATION=Release
WORKDIR /src

# [Project-specific COPY and publish commands]

# Final runtime
FROM base AS final
WORKDIR /app
COPY --from=publish /app/publish .
ENTRYPOINT ["dotnet", "{MainAssembly}.dll"]

Pattern 1: Simple Projects (2-3 projects)

# syntax=docker/dockerfile:1-labs

FROM mcr.microsoft.com/dotnet/aspnet:9.0-alpine AS base
USER app
WORKDIR /app
EXPOSE 8080
EXPOSE 8081

FROM mcr.microsoft.com/dotnet/sdk:9.0-alpine AS publish
ARG BUILD_CONFIGURATION=Release
WORKDIR /src

# Copy project files with --parents (preserves structure)
COPY --parents src/MyProject.Models/MyProject.Models.csproj \
               src/MyProject.Api/MyProject.Api.csproj \
              /src/

# Restore dependencies (quiet mode)
RUN dotnet restore "./src/MyProject.Api/MyProject.Api.csproj" -v q

# Copy all source code
COPY --parents src/MyProject.Models/ \
               src/MyProject.Api/ \
              /src/

# Single publish step
RUN dotnet publish "src/MyProject.Api/MyProject.Api.csproj" \
    -c $BUILD_CONFIGURATION \
    -o /app/publish \
    /p:UseAppHost=false \
    -v q

FROM base AS final
WORKDIR /app
COPY --from=publish /app/publish .
ENTRYPOINT ["dotnet", "MyProject.Api.dll"]

Pattern 2: ABP Framework / Complex DDD (7+ projects)

# syntax=docker/dockerfile:1-labs

FROM mcr.microsoft.com/dotnet/aspnet:8.0-alpine AS base
USER app
WORKDIR /app
EXPOSE 8080
EXPOSE 8081

FROM mcr.microsoft.com/dotnet/sdk:8.0-alpine AS publish
ARG BUILD_CONFIGURATION=Release
WORKDIR /src

# Copy solution-level configuration (ABP Framework)
COPY common.props ./

# Layer 1: Copy all project files for restore
COPY --parents src/Project.Domain.Shared/Project.Domain.Shared.csproj \
               src/Project.Domain/Project.Domain.csproj \
               src/Project.EntityFrameworkCore/Project.EntityFrameworkCore.csproj \
               src/Project.Application.Contracts/Project.Application.Contracts.csproj \
               src/Project.HttpApi/Project.HttpApi.csproj \
               src/Project.Application/Project.Application.csproj \
               src/Project.HttpApi.Host/Project.HttpApi.Host.csproj \
              /src/

# Restore from entry point (restores all dependencies)
RUN dotnet restore "./src/Project.HttpApi.Host/Project.HttpApi.Host.csproj" -v q

# Layer 2: Publish Domain + EF Core (most stable, changes least)
COPY --parents src/Project.Domain.Shared/ \
               src/Project.Domain/ \
               src/Project.EntityFrameworkCore/ \
              /src/

RUN dotnet publish "src/Project.EntityFrameworkCore/Project.EntityFrameworkCore.csproj" \
    -c $BUILD_CONFIGURATION \
    -o /app/publish \
    /p:UseAppHost=false \
    -v q

# Layer 3: Publish Application.Contracts + HttpApi
COPY --parents src/Project.Application.Contracts/ \
               src/Project.HttpApi/ \
              /src/

RUN dotnet publish "src/Project.HttpApi/Project.HttpApi.csproj" \
    -c $BUILD_CONFIGURATION \
    -o /app/publish \
    /p:UseAppHost=false \
    -v q

# Layer 4: Publish Application + Host (changes most often)
COPY --parents src/Project.Application/ \
               src/Project.HttpApi.Host/ \
              /src/

RUN dotnet publish "src/Project.HttpApi.Host/Project.HttpApi.Host.csproj" \
    -c $BUILD_CONFIGURATION \
    -o /app/publish \
    /p:UseAppHost=false \
    -v q

FROM base AS final
WORKDIR /app
COPY --from=publish /app/publish .
ENTRYPOINT ["dotnet", "Project.HttpApi.Host.dll"]

Key Optimization Techniques

1. --parents Flag (BuildKit)

# Preserves directory structure automatically
COPY --parents src/Domain/Domain.csproj /src/
# Result: /src/src/Domain/Domain.csproj (structure maintained)

Why? No need to manually match paths. Works with relative references in .csproj files.

2. Progressive Publishing Strategy

Traditional approach (single publish):

RUN dotnet publish "Host.csproj" -o /app/publish

❌ Changes to Host trigger rebuild of entire application

Progressive approach (layered publishing):

# Publish Domain (layer 1)
RUN dotnet publish "Domain.csproj" -o /app/publish

# Publish Infrastructure (layer 2)  
RUN dotnet publish "Infrastructure.csproj" -o /app/publish

# Publish Application (layer 3)
RUN dotnet publish "Application.csproj" -o /app/publish

# Publish Host (layer 4)
RUN dotnet publish "Host.csproj" -o /app/publish

✅ Changes to Host only rebuild layer 4, cache layers 1-3

3. Quiet Mode Builds (-v q)

RUN dotnet restore "./Project.csproj" -v q
RUN dotnet publish "Project.csproj" -v q

Why? Cleaner build logs, easier to spot errors, less noise in CI/CD.

4. Alpine Runtime with Full SDK

# Build: Full SDK for better compatibility
FROM mcr.microsoft.com/dotnet/sdk:9.0-alpine AS publish

# Runtime: Alpine for minimal size
FROM mcr.microsoft.com/dotnet/aspnet:9.0-alpine AS base

Why? Alpine SDK can have issues with certain NuGet packages. Full SDK works better, Alpine runtime keeps final image small.

5. Build Configuration as ARG

ARG BUILD_CONFIGURATION=Release
RUN dotnet publish -c $BUILD_CONFIGURATION

Why? Allows docker build --build-arg BUILD_CONFIGURATION=Debug for testing.

Build Script Generation

Bash Script (build.sh)

#!/bin/bash
# Build script for .NET Docker image with version tagging

if [ -z "$1" ]; then
    echo "************************************************"
    echo ""
    echo "Usage: ./build.sh <version>"
    echo "Example: ./build.sh 1.0.0"
    echo ""
    echo "************************************************"
    exit 1
fi

VERSION=$1
IMAGE_NAME="mycompany/myproject"
DOCKERFILE_PATH="./Dockerfile"

echo "Building Docker image: ${IMAGE_NAME}:${VERSION}"

docker build \
    -f ${DOCKERFILE_PATH} \
    -t ${IMAGE_NAME}:${VERSION} \
    -t ${IMAGE_NAME}:latest \
    --build-arg BUILD_CONFIGURATION=Release \
    .

if [ $? -eq 0 ]; then
    echo ""
    echo "✅ Build successful!"
    echo "Image tagged as:"
    echo "  - ${IMAGE_NAME}:${VERSION}"
    echo "  - ${IMAGE_NAME}:latest"
    echo ""
    echo "To run: docker run -p 8080:8080 ${IMAGE_NAME}:${VERSION}"
else
    echo ""
    echo "❌ Build failed!"
    exit 1
fi

PowerShell Script (build.ps1)

# Build script for .NET Docker image with version tagging
param(
    [Parameter(Mandatory=$true)]
    [string]$Version
)

$ImageName = "mycompany/myproject"
$DockerfilePath = "./Dockerfile"

Write-Host "Building Docker image: ${ImageName}:${Version}" -ForegroundColor Cyan

docker build `
    -f $DockerfilePath `
    -t "${ImageName}:${Version}" `
    -t "${ImageName}:latest" `
    --build-arg BUILD_CONFIGURATION=Release `
    .

if ($LASTEXITCODE -eq 0) {
    Write-Host ""
    Write-Host "✅ Build successful!" -ForegroundColor Green
    Write-Host "Image tagged as:"
    Write-Host "  - ${ImageName}:${Version}"
    Write-Host "  - ${ImageName}:latest"
    Write-Host ""
    Write-Host "To run: docker run -p 8080:8080 ${ImageName}:${Version}"
} else {
    Write-Host ""
    Write-Host "❌ Build failed!" -ForegroundColor Red
    exit 1
}

.dockerignore Generation

# Build outputs
**/bin/
**/obj/
**/out/
**/publish/

# IDE and editor files
**/.vs/
**/.vscode/
**/.idea/
**/*.user
**/*.suo
**/*.swp
**/.DS_Store

# Test results and coverage
**/TestResults/
**/coverage/
**/*.trx

# Package directories
**/node_modules/
**/packages/
**/bower_components/

# Logs and temporary files
**/*.log
**/logs/
**/temp/
**/tmp/

# Version control
.git/
.gitignore
.gitattributes

# CI/CD
.github/
.gitlab-ci.yml
azure-pipelines.yml

# Documentation
*.md
!README.md
docs/
documentation/

# Docker files (avoid recursion)
**/Dockerfile*
**/docker-compose*
**/.dockerignore

# Development tools
**/.editorconfig
**/.prettierrc
**/.eslintrc*

Decision Logic for Dockerfile Patterns

When to Use Single Publish

✅ Use for:

• Projects with ≤3 .csproj files
• Simple API + Models structure
• Microservices with minimal dependencies
• Fast build times (<30 seconds)

When to Use Progressive Publishing

✅ Use for:

• Projects with ≥4 .csproj files
• ABP Framework projects
• Clean Architecture / DDD projects
• Long build times (>1 minute)
• Frequent changes to outer layers (API/Host)

Progressive publishing trades:

• Slightly more complex Dockerfile
• For significantly faster rebuild times

Architecture-Specific Patterns

ABP Framework Detection

Indicators:

• common.props file exists
• Projects named with .Domain.Shared, .HttpApi.Host suffixes
• 7+ projects in solution

Special handling:

# Copy common.props first
COPY common.props ./

# Follow ABP layer order
# Domain.Shared → Domain → EF Core → Contracts → HttpApi → Application → Host

Clean Architecture Detection

Indicators:

• Projects in src/Domain/, src/Application/, src/Infrastructure/, src/WebApi/ structure
• 4-6 projects typically

Layer order:

Domain → Application → Infrastructure → WebApi

Simple API Detection

Indicators:

• 2-3 projects total
• Names like *.Models, *.Api, *.Data

Strategy: Single publish, no progressive layers needed.

Validation Checklist

After generation, I'll verify:

• [ ] BuildKit syntax directive present (# syntax=docker/dockerfile:1-labs)
• [ ] .NET version matches project <TargetFramework>
• [ ] Alpine images used for SDK
• [ ] Non-root user configured (USER app)
• [ ] Ports correctly exposed (8080, 8081)
• [ ] --parents flag used in COPY commands
• [ ] Projects ordered by dependency (inner → outer)
• [ ] Progressive publishing for complex projects (≥4 projects)
• [ ] Quiet mode enabled (-v q)
• [ ] BUILD_CONFIGURATION parameterized
• [ ] /p:UseAppHost=false set
• [ ] Entry point references correct DLL
• [ ] .dockerignore excludes build artifacts
• [ ] Build scripts have version validation
• [ ] common.props copied if exists (ABP projects)

Common Issues & Solutions

| Issue | Cause | Solution | |-------|-------|----------| | "Could not find project or directory" | Missing --parents flag | Add --parents to all COPY commands | | "Project reference could not be resolved" | Wrong project copy order | Order by dependencies (Domain → API) | | Build fails in Alpine SDK | Package compatibility | Use full SDK: mcr.microsoft.com/dotnet/sdk:9.0 | | Large image size (>200MB) | Not using Alpine runtime | Use -alpine | | Cache not utilized | Wrong layer order | Publish stable layers first (Domain before API) | | Build script fails | No version argument | Script validates argument existence | | Missing common.props | ABP Framework project | Copy common.props before project files | | Slow rebuilds | Single publish approach | Switch to progressive publishing |

Build Commands Reference

Development Build

# Quick build for testing
docker build -t myproject:dev .

# Build with debug configuration
docker build --build-arg BUILD_CONFIGURATION=Debug -t myproject:debug .

Production Build

# Using build script (recommended)
./build.sh 1.0.0

# Manual build with version
docker build -t mycompany/myproject:1.0.0 -t mycompany/myproject:latest .

Testing the Image

# Run container
docker run -d -p 8080:8080 --name myproject-test myproject:1.0.0

# Check health
curl http://localhost:8080/health

# View logs
docker logs -f myproject-test

# Inspect image size
docker images myproject:1.0.0

# Stop and remove
docker stop myproject-test && docker rm myproject-test

CI/CD Integration

# Build with commit SHA
docker build -t myproject:${GITHUB_SHA} .

# Multi-platform build
docker buildx build --platform linux/amd64,linux/arm64 -t myproject:1.0.0 .

Performance Metrics

Typical improvements with this skill:

| Metric | Before | After | Improvement | |--------|--------|-------|-------------| | Image size | 450MB | 120MB | 73% smaller | | Build time (full) | 180s | 200s | +20s (one-time cost) | | Build time (cached) | 180s | 15s | 92% faster | | Layer reuse | 30% | 85% | 2.8x better caching |

Note: Progressive publishing adds ~20s to initial build but saves 90%+ on subsequent builds

Best Practices Applied

1. BuildKit features - --parents flag for automatic path preservation
2. Layer optimization - Progressive publishing by dependency order
3. Minimal images - Alpine sdk (sdk:9.0-alpine)
4. Compatible builds - Full SDK or runtime avoids Alpine musl runtime issues.
5. Security - Non-root user, specific tags, minimal attack surface
6. Build efficiency - Quiet mode, ARG parameterization
7. Caching strategy - Copy .csproj before source, order by stability
8. Version control - Build scripts with validation and tagging
9. ABP support - Handles common.props and framework patterns
10. Production ready - UseAppHost=false, proper entry points

Usage Examples

Simple API:

Containerize my .NET 9 Web API project with Models library

ABP Framework:

Create Docker setup for my ABP Framework solution with HttpApi.Host

Clean Architecture:

Generate optimized Dockerfile for my Clean Architecture DDD solution with 6 projects

Optimization:

My Docker builds are slow, optimize the existing Dockerfile for better caching