zstd layers for quantecon-build, plus a tag input for test-container.yml

[mmcky]

From #92 Phase 1: zstd-compressed layers for quantecon-build in build-containers.yml, worth −20–35s per PR pull (measured pull is 129s, 75% of a warm lecture-dp run).

Neither docker/build-push-action call sets outputs: (.github/workflows/build-containers.yml:66-74, :108-116), so both push default gzip.

• Add a tag input to test-container.yml — its workflow_dispatch: takes no inputs: (:6-11) and :latest is hardcoded at :33, :99
• Push a :zstd tag and validate it with the smoke suite before :latest moves — :latest reaches consumers ungated by any release (see Optimize preview builds: phased plan for the fastest PR previews (tracking) #92's note against Release gating: require a green canary before v0 moves (adds a v0-next staging tag) #135)
• Flip the build to zstd layers once validated