refactor(docker): consolidate dual Dockerfile into single BuildKit-cached build

[h0lybyte]

Summary

• Remove Dockerfile.base and BUILD_BASE arg pattern across all apps (axum-kbve, discordsh, cryptothrone, mc)
• Single Dockerfile with cargo-chef + BuildKit --mount=type=cache handles dependency caching inline
• On arc-runner, switch from GHCR registry cache to local PVC-backed cache (/home/runner/_cache/docker/) for persistent layer caching across pod restarts
• Remove build_base CI job, has_base/base_image config outputs, basex/base Nx targets, and _base file alteration filters

Changes

• 4 Dockerfile.base files deleted (-388 lines)
• 4 Dockerfiles simplified — ARG BUILD_BASE removed, FROM ${BUILD_BASE} → FROM foundation
• 4 project.json files — basex/base targets removed, BUILD_BASE build-args stripped, axum-kbve CI uses type=local cache
• ci-docker.yml — build_base job removed, has_base/base_image config dropped
• utils-file-alterations.yml — dead _base filter outputs removed

Test plan

• Verify nx container axum-kbve builds locally (inline foundation stages)
• Verify CI docker pipeline runs without build_base job
• Confirm arc-runner PVC cache populates on first build and accelerates subsequent builds

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None