Fix lables to not have - character

Annoyingly, the `docker inspect --format` syntax doesn't support
- characters in labels.

So rename `build-date` to `created`, and `vcs-ref` to gitref

Dockerfile

@@ -34,8 +34,8 @@ RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
 # built from
 ARG BASE_BUILD_DATE
 ARG BASE_GITREF
-LABEL org.opensafely.base.build-date=$BASE_BUILD_DATE \
-      org.opensafely.base.vcs-ref=$BASE_GITREF
+LABEL org.opensafely.base.created=$BASE_BUILD_DATE \
+      org.opensafely.base.gitref=$BASE_GITREF
 
 FROM base-docker as base-action