Clean up the build cache with docker buildx prune

Documented steps

1. Run 'docker buildx prune' to remove BuildKit build cache for the current builder; it prompts for confirmation, so add -f/--force to skip the prompt in scripts.
2. Add -a/--all to also clear internal helper images and frontend images set via the #syntax directive or BUILDKIT_SYNTAX build arg.
3. Selectively delete records with --filter using selector syntax '<key><op><value>' where op is = (equal), != (not equal), or ~= (regex). Multiple --filter flags are ANDed.
4. Useful filters: 'until=24h' or 'until=2h30m' (H/M/S units) to keep recently-used records, 'description~=golang' to match by description substring, 'parents=<id>;<id>' to target children of specific IDs, 'type=exec.cachemount' or 'type=source.local' to target cache mounts or local sources, and 'inuse'/'mutable'/'immutable'/'shared'/'private' for state-based selection.
5. Enforce cache size rather than age with --max-used-space=2gb (trim LRU records until under the cap), --min-free-space=2gb (keep at least that much disk free), or --reserved-space=2gb (always keep that much for cache). Units are human-readable like 128mb/2gb, case-insensitive.
6. Scope to a different builder with --builder <name>.
7. Official docs: https://docs.docker.com/reference/cli/docker/buildx/prune/ and https://docs.docker.com/build/cache/garbage-collection/