Feature request: dedicated environment variable for cache mount location

[rassie]

Is your feature request related to a problem?

When using RUN --mount=type=cache, buildah stores cache mounts at $TMPDIR/buildah-cache-<uid>/. The only way to control this location is via TMPDIR or image_copy_tmp_dir in containers.conf.

Both settings affect all buildah temporary operations (SSH agent sockets, heredoc files, build contexts, downloaded URLs, container bundles), not just cache mounts.

In CI/CD environments this forces a choice:

• Redirect all temp operations to persistent storage (wasteful, stores ephemeral data)
• Keep default TMPDIR and lose cache mount persistence between jobs

Describe the solution you'd like

A dedicated environment variable (e.g. BUILDAH_CACHE_DIR) that controls only the location of --mount=type=cache mounts, independent of TMPDIR.

Priority order:

1. BUILDAH_CACHE_DIR (new, cache-specific)
2. TMPDIR (existing behavior)
3. image_copy_tmp_dir from containers.conf
4. /var/tmp (default)

Describe alternatives you've considered

1. Setting TMPDIR globally in runner config - affects all applications in all CI jobs, not just buildah. Node.js, Python, Java, and other tools would write temp files to persistent storage, causing unnecessary I/O and storage bloat. Also risks breaking tools that expect TMPDIR to be ephemeral.
2. Using image_copy_tmp_dir - still affects all buildah temp operations, not just cache mounts
3. Per-job TMPDIR in CI - requires job authors to know infrastructure details and remember to set it

Additional context

Use case: GitLab CI with Kubernetes runner

• Jobs run in ephemeral pods
• Shared PVC at /buildah-cache for cache persistence
• Want ephemeral temp files to stay on local/tmpfs storage

Proposed implementation in internal/volumes/volumes.go:

func CacheParent() string {
    if cacheDir, ok := os.LookupEnv("BUILDAH_CACHE_DIR"); ok {
        return filepath.Join(cacheDir, buildahCacheDir+"-"+strconv.Itoa(unshare.GetRootlessUID()))
    }
    return filepath.Join(tmpdir.GetTempDir(), buildahCacheDir+"-"+strconv.Itoa(unshare.GetRootlessUID()))
}