build-container: add concurrency group to prevent stale deploys

[oalders]

Closes #3733

Problem

build-container.yml had no concurrency group, so two overlapping master builds could finish seconds apart with the slower one deploying an older commit — rolling back a newer deploy.

Change

Added a workflow-level concurrency block:

• group: ${{ github.workflow }}-${{ github.ref }} — one group per ref, so overlapping master pushes share a group and cancel each other while PR/staging/prod builds stay independent.
• cancel-in-progress: true — supersedes older in-progress builds; the newest commit to enter the group wins the deploy.

Used ${{ github.workflow }}-${{ github.ref }} (GitHub idiom) instead of a hardcoded build-container- prefix so it survives a workflow rename; behavior is identical.

Tradeoff

Intermediate master commits in a burst no longer get images (superseded builds are cancelled). Rollbacks target previously deployed tags, which are unaffected. This only makes the race rare; the ordering guarantee belongs in set-image.yml (metacpan/metacpan-k8s#206).

Verification

• YAML parses; on: triggers (push/pull_request/workflow_dispatch) intact.
• Reviewed for concurrency semantics: cancel-in-progress can only cancel older in-progress runs, never the newest, and the single-job workflow leaves no half-deployed state.

Not auto-marked ready: touches .github/workflows/**, so it is left in draft for a human review.

🤖 Generated with Claude Code · Opus 4.8

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.80%. Comparing base (b4caf5d) to head (8ee79df).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3734   +/-   ##
=======================================
  Coverage   76.80%   76.80%           
=======================================
  Files          71       71           
  Lines        2522     2522           
  Branches      360      360           
=======================================
  Hits         1937     1937           
  Misses        455      455           
  Partials      130      130           

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