dev.Dockerfile: use cache mounts to speed up rebuilding

[ZZiigguurraatt]

The branch in #1161 was accidentally deleted and the PR automatically closed, so this PR re-opens it with a new copy of the branch.

• dev.Dockerfile now uses
  cache mounts
  to cache the GOMODCACHE and GOCACHE directories so that dependencies don't
  need to be re-downloaded and re-built every time the image is re-created.
  As a result of this change, dev.Dockerfile now requires
  BuildKit to build. When using
  docker build, this can be enabled by setting the environmental variable
  DOCKER_BUILDKIT=1. BuildKit also does not unnecessarily rebuild images when
  the build context is a remote git repository because COPY layers are more
  smartly compared to cache.

Would like some feedback on this change. Not sure if it might have some unintended consequences that I have not thought of, but it really speeds things up a lot.

[gemini-code-assist]

Summary of Changes

Hello @ZZiigguurraatt, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request focuses on enhancing the build efficiency of the dev.Dockerfile by integrating Docker BuildKit's cache mount capabilities. By caching the GOMODCACHE and GOCACHE directories, the changes aim to drastically reduce the time and resources required for subsequent image rebuilds, as Go dependencies will no longer need to be re-downloaded and re-compiled from scratch. This optimization streamlines the development workflow, though it introduces a dependency on BuildKit for the Docker build process.

Highlights

• dev.Dockerfile Performance Improvement: Implemented Docker cache mounts for GOMODCACHE and GOCACHE in dev.Dockerfile to significantly speed up image rebuilding by caching Go dependencies.
• BuildKit Requirement: The updated dev.Dockerfile now requires Docker BuildKit to be enabled (via DOCKER_BUILDKIT=1) to utilize the new cache mount features.
• Improved COPY Layer Caching: BuildKit's smarter comparison of COPY layers further optimizes rebuilds, especially when the build context is a remote git repository.
• PR Re-opening: This pull request re-opens a previously deleted and automatically closed branch, bringing back the proposed changes.

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[gemini-code-assist]

Code Review

This pull request introduces a great optimization to the dev.Dockerfile by using cache mounts for Go modules and build cache, which will significantly speed up development builds. The change is well-documented in both the Dockerfile and the release notes. I have one suggestion to further improve Docker layer caching by separating the installation of apk packages into its own RUN layer. This is a minor optimization on top of your existing improvements.

[lightninglabs-deploy]

@ZZiigguurraatt, remember to re-request review from reviewers when ready