fix(ci): tolerate Docker Hub login failure (issue #82)

.changeset/issue-82-tolerate-dockerhub-login-failure.md

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+---
+bump: patch
+---
+
+ci(release): tolerate Docker Hub login failure so a single expired
+DOCKERHUB_TOKEN no longer takes down the entire release workflow
+(issue #82). Every "Log in to Docker Hub" step now uses
+`continue-on-error: true` and is followed by a "Check Docker Hub
+login" step that emits a clear `::warning` annotation pointing at
+the rotation runbook in `README.md` and `docs/case-studies/issue-82`.
+GHCR pushes proceed on their existing credentials when Docker Hub is
+unavailable.
+
+ci(release): free ~30 GB of disk space before `docker-build-test` so
+the PR-CI smoke job stops failing with `no space left on device`
+while building the JS -> essentials -> 11 language images -> full-box
+chain on a single ubuntu-24.04 runner. Mirrors the existing
+`jlumbroso/free-disk-space` step in `docker-build-push` (issue #41).
+
+ci(release): parallelize the PR test matrix and isolate every Docker
+image build on its own VM (issue #82). The single sequential
+`docker-build-test` job is replaced by a chain of parallel matrix jobs:
+`pr-test-js` (1 VM) -> `pr-test-essentials` (1 VM) ->
+`pr-test-language` (matrix x 11 languages, parallel) ->
+`pr-test-full` (1 VM, builds the full chain locally because the
+`full-box` Dockerfile uses `COPY --from=*-stage`), with
+`pr-test-dind` (matrix x 14 variants, parallel) running alongside
+`pr-test-full` once `pr-test-essentials` finishes. A
+`docker-build-test` aggregator job preserves the existing
+branch-protection check name. Every build job (15 jobs:
+`pr-test-*`, `build-{js,essentials,languages,dind}-{amd64,arm64}`,
+`docker-build-push{,-arm64}`) now runs `jlumbroso/free-disk-space@main`
+before its first build step. PR-test jobs use plain `docker build`
+against the host Docker daemon (not buildx's `docker-container`
+driver) so that subsequent `FROM box-js` / `FROM box-essentials`
+references resolve against the locally-built images instead of
+trying to pull from `docker.io`.
+
+fix(kotlin): install Java 21 LTS alongside Kotlin in the standalone
+`box-kotlin` image (issue #82). `kotlinc` is a shell wrapper around
+`java`, so the language-only image previously failed
+`docker run --rm box-kotlin kotlin -version` with
+`line 102: java: command not found`. The full-box image was unaffected
+because Java was supplied by the `box-java` stage. Now
+`ubuntu/24.04/kotlin/install.sh` installs Java via SDKMAN before
+Kotlin, making the standalone `box-kotlin` image self-sufficient and
+unblocking the new per-language `pr-test / kotlin` matrix job.