Splits the runtime apt installs (gdal-bin, snmp, libsnmp40, locales,
BEAM runtime libs) into k8s/Dockerfile.base, hosted at
codeberg.org/gmcintire/towerops-base:latest. The app Dockerfile now
does FROM that base instead of re-installing gdal (~500 MB) on every
push.
The new build-base workflow rebuilds the base image only when
k8s/Dockerfile.base or the workflow itself changes, weekly via cron
(Sundays 06:00 UTC, with CACHE_BUST=<ISO week> to force apt-get update
on a week boundary), or via workflow_dispatch.
Production workflow now uses buildx + does docker login before the
build so it can pull the private base image.
