Dockerfile & Image Optimization

Writing a Dockerfile

A Dockerfile is a text file containing instructions to build a Docker image. Here's a simple example for a .NET web app:

# Use SDK image to build the app
FROM mcr.microsoft.com/dotnet/sdk:8.0 AS build
WORKDIR /src
COPY . .
RUN dotnet publish -c Release -o /app/publish

# Use runtime image to run the app
FROM mcr.microsoft.com/dotnet/aspnet:8.0
WORKDIR /app
COPY --from=build /app/publish .
ENTRYPOINT ["dotnet", "MyWebApp.dll"]

This is an example of a multi-stage build, which is great for keeping images small and clean.

Understanding Image Layers

Each instruction in a Dockerfile creates a new layer. Docker caches these layers to speed up rebuilds.

• Changing an early layer (e.g., COPY) will invalidate all subsequent layers.
• Group related instructions (e.g., RUN commands) to reduce layers.

Multi-Stage Builds

Multi-stage builds allow you to separate the build environment from the runtime image, resulting in smaller images:

• Use one stage to compile/build the app.
• Use a second (smaller) stage to run the final app.

This ensures only the compiled code and required binaries go into production images.

Reducing Image Size

• Use official minimal base images like alpine where possible.
• Combine RUN instructions to reduce layers and intermediate cache:

  RUN apt-get update && apt-get install -y curl && rm -rf /var/lib/apt/lists/*

  Copy
• Use .dockerignore to exclude unnecessary files from being copied into the image.

Best Practices for Dockerfile Optimization

• Specify exact base image versions (avoid latest) for stability.
• Minimize the number of layers and keep instructions concise.
• Use ENV to set environment variables consistently.
• Scan images for vulnerabilities using tools like docker scan or Snyk.
• Clean up temp files and package caches during image build.

Writing optimized Dockerfiles ensures leaner, faster, and more secure container images. Multi-stage builds, cache management, and smart layering are essential techniques in professional Docker development.