Docker build cache and layers

Topic: Containers core

Docker caches layers; change one line and everything after rebuilds. Put rarely changed steps first and frequently changed steps last. Use this when optimizing build speed.

Intent: How-to

Quick answer

  • Order matters. COPY package.json before COPY . and RUN install so dependency layer is cached when only code changes.
  • Each RUN, COPY, ADD creates a layer. Combine RUN commands where it makes sense; avoid unnecessary layers.
  • Build cache can be invalidated by parent change or --no-cache. Use .dockerignore to avoid copying files that change often and bust cache.

Prerequisites

Steps

  1. Order for cache

    Put base and dependency install first; copy source and build last. So code changes do not invalidate dependency layer.

  2. Use .dockerignore

    Exclude .git, tmp, local env files so COPY . does not pull in unnecessary or changing files.

  3. Verify

    Change one file; rebuild. Only layers after the change should rebuild. Use docker build --no-cache to test full rebuild.

Summary

Order Dockerfile so stable steps come first; use .dockerignore; verify cache with incremental builds.

Prerequisites

Steps

Step 1: Order for cache

Dependencies first; source and build steps last.

Step 2: Use .dockerignore

Ignore .git, tmp, and env files to keep COPY cache stable.

Step 3: Verify

Rebuild after small change; confirm only later layers rebuild.

Verification

  • Code-only change triggers minimal rebuild.

Troubleshooting

Cache busted — Check order and .dockerignore. Slow build — Use multi-stage; reduce layer count.

Next steps

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