How to Force Docker Build to Overwrite Files During COPY Operations in Dockerfile


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When building Docker images, the COPY instruction follows Docker's layer caching mechanism which can lead to unexpected behavior when overwriting files. The issue stems from how Docker handles the build cache:

# This won't work as expected due to layer caching
COPY config/file1.yml /app/config/settings.yml
COPY config/file2.yml /app/config/settings.yml  # This change won't take effect

Here are three effective approaches to force file overwrites during Docker builds:

Method 1: Using Multiple Build Stages

# Stage 1 - Base configuration
FROM alpine as base
COPY config/default/ /app/config/

# Stage 2 - Override specific files
FROM base as production
COPY config/prod/ /app/config/  # This will properly overwrite

Method 2: Atomic Directory Copy

# Combine all files in a temp directory first
RUN mkdir -p /tmp/config
COPY config/default/ /tmp/config/
COPY config/prod-overrides/ /tmp/config/
RUN cp -rf /tmp/config/* /app/config/ && rm -rf /tmp/config

Method 3: Using --no-cache Build Option

# This forces complete rebuild but isn't ideal for CI/CD
docker build --no-cache -t myapp .

For more complex scenarios, consider using build arguments to select files:

ARG ENV=dev
COPY config/${ENV}/ /app/config/
COPY config/common/ /app/config/  # Common files

Then build with:

docker build --build-arg ENV=prod -t myapp .

While these solutions work, be mindful of:

  • Build time impact when invalidating cache
  • Image layer bloat from multiple copy operations
  • Debugging complexity in multi-stage builds

When working with Docker's COPY instruction, you might encounter a situation where subsequent COPY commands don't overwrite previously copied files, even when they target the same destination path. This behavior occurs because Docker's build cache treats each COPY instruction independently.

Consider this Dockerfile scenario:

FROM alpine:latest
WORKDIR /app
COPY config/file1.yml ./config/thatfile.yml
COPY config/file2.yml ./config/thatfile.yml

After building, you'll find that /app/config/thatfile.yml contains the contents of file1.yml, not file2.yml as you might expect.

Method 1: Using Multiple Build Stages

FROM alpine:latest as stage1
WORKDIR /app
COPY config/file1.yml ./config/thatfile.yml

FROM alpine:latest as stage2
WORKDIR /app
COPY config/file2.yml ./config/thatfile.yml

FROM alpine:latest
WORKDIR /app
COPY --from=stage2 /app/config/thatfile.yml ./config/

Method 2: Leveraging .dockerignore

Create a .dockerignore file to exclude the original file before copying:

config/thatfile.yml

Method 3: Using RUN Commands with rm

FROM alpine:latest
WORKDIR /app
COPY config/file1.yml ./config/thatfile.yml
RUN rm -f ./config/thatfile.yml
COPY config/file2.yml ./config/thatfile.yml

For more complex scenarios where you need to conditionally overwrite files based on build arguments:

FROM alpine:latest
WORKDIR /app
ARG CONFIG_VERSION=default
COPY config/file1.yml ./config/thatfile.yml
RUN if [ "$CONFIG_VERSION" != "default" ]; then \
    rm -f ./config/thatfile.yml && \
    COPY config/file2.yml ./config/thatfile.yml; \
    fi
  • Consider using environment variables instead of file replacements when possible
  • For complex configurations, use a configuration management tool inside the container
  • Document your overwrite patterns clearly in the Dockerfile comments

Each solution has different implications for build time and cache efficiency. The multi-stage approach generally provides the best balance between correctness and performance, while the RUN+rm method is simplest but may impact caching.