add cache key

[luke-lombardi]

Summary by cubic

Add in-memory caching for image builds to reuse prior results and avoid redundant work, speeding up repeated builds.

• New Features
  • Cache image build results keyed by config; checks cache before build and stores results after an exists/build success.
  • Default TTL is 5 minutes; override with BEAM_IMAGE_BUILD_CACHE_TTL_SECONDS or disable with BEAM_DISABLE_IMAGE_BUILD_CACHE.
  • Skips caching failed builds and drops expired entries.

^{Written for commit a8631b5. Summary will update on new commits.}