Pre-builds the runtime base (wgrib2 + g2c + cdo + gdal-bin + locale +
runtime apt deps) into git.mcintire.me/graham/prop-base. The base
workflow is path-filtered to fire only on Dockerfile.base /
build-base.yaml changes, so version bumps stay deterministic and
explicit.
Tag convention: `wgrib2-<v>-g2c-<v>-<unixts>` for traceability +
rollback, plus `:latest` for everyday consumption.
This commit only adds the new files — the app Dockerfile + build.yaml
still build wgrib2 from source. A follow-up will swap the app
Dockerfile to FROM prop-base:latest once this image lands in the
registry.
