Why Docker Builds Are Slow in CI and How to Fix Layer Caching

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Docker builds that complete in seconds locally can take over 10 minutes in CI because hosted runners are ephemeral fresh VMs with no cached layers, forcing every step to re-execute from scratch. The root cause is almost never the base image size, but rather cache misses caused by poor Dockerfile layer ordering. Placing a COPY . . instruction before dependency installation means any code change invalidates the dependency layer, preventing cache reuse entirely. The fix involves restructuring the Dockerfile so rarely changing files like lockfiles are copied and dependencies installed before application source code is added. Attaching an external cache backend, such as GitHub Actions' built-in cache, and using the --progress=plain flag to audit which steps are actually cached completes the solution.

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