Add consistent OCI labels to all container Dockerfiles

Every container now carries title, description, version, source, and
vendor labels per the OCI image spec. Version is derived from the
existing CONTAINER_APP_VERSION ARG at build time.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

containers/devpi/Dockerfile

@@ -4,6 +4,12 @@ FROM python:3.12-slim
 
 ARG CONTAINER_APP_VERSION
 ARG DEVPI_SERVER_VERSION=${CONTAINER_APP_VERSION}
+
+LABEL org.opencontainers.image.title="devpi"
+LABEL org.opencontainers.image.description="devpi PyPI server and caching proxy"
+LABEL org.opencontainers.image.version="${CONTAINER_APP_VERSION}"
+LABEL org.opencontainers.image.source="https://forge.eblu.me/eblume/blumeops"
+LABEL org.opencontainers.image.vendor="blumeops"
 ARG DEVPI_WEB_VERSION=5.0.1
 
 # Install devpi-server and devpi-web