Move the init-container loader image build from cameleer-server CI into
this repo so all sidecar/infra image builds (runtime-base, postgres,
clickhouse, traefik, logto, and now runtime-loader) live in one place.
The loader is consumed by cameleer-server's DockerRuntimeOrchestrator as
a per-replica init container that fetches the tenant JAR from a signed
URL into a named volume before the main container starts. Source +
Dockerfile copied verbatim from cameleer-server@c2efb7fb (the image with
the volume-permission fix). The published tag path is unchanged
(gitea.siegeln.net/cameleer/cameleer-runtime-loader:latest), so running
tenant servers continue pulling the same image.
Build step matches the runtime-base/postgres/clickhouse/traefik pattern
(unconditional rebuild on every push, sha + branch tags, --provenance=false
for Gitea). cameleer-server will follow up with a commit removing its
loader-build step and switching its LoaderHardeningIT to pull the
published image instead of building from a local Dockerfile.
