The registry sits behind a reverse proxy that 413s large blob pushes,
so templates/web/Dockerfile's oversized RUN blocks (apt installs,
rustup targets, cargo installs) are broken up so no single layer is
too big to push.
Also adds templates/<name>/VERSION (starting at "1" for all six
templates) and has the provision job's push step look up whether that
version is already pushed before running coder templates push, since
the workflow triggers on any change under templates/** and previously
reprovisioned every template on every push, not just the one that
changed.
