Notes from a 2 AM On-Call: Why Gitea's Docker Build Ate 40 Minutes (And the 3-Line BuildKit Fix)

A recent issue with Gitea's continuous integration pipeline revealed a significant slowdown in Docker build times, jumping from 8 minutes to over 40 minutes despite no code changes. The root cause was a subtle ordering bug in the Dockerfile where ARG declarations appeared before the WORKDIR, invalidating BuildKit's layer fingerprinting due to changing variable values. Moving these ARG declarations after the COPY instructions restored build times to nearly 8 minutes and improved cache hit rates, highlighting the importance of ARG placement for efficient Docker builds.

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