specify user in rootless container numerically

With kubernetes' PodSecurityPolicy set to runAsNonRoot it will not allow starting the container.  The error message is
```
Error: container has runAsNonRoot and image has non-numeric user (git), cannot verify user is non-root
```
The `USER` directive has to be numerical for that to work.

Author: ibotty

Dockerfile.rootless

@@ -56,7 +56,7 @@ COPY docker/rootless /
 COPY --from=build-env --chown=root:root /go/src/code.gitea.io/gitea/gitea /usr/local/bin/gitea
 COPY --from=build-env --chown=root:root /go/src/code.gitea.io/gitea/environment-to-ini /usr/local/bin/environment-to-ini
 
-USER git:git
+USER 1000:1000
 ENV GITEA_WORK_DIR /var/lib/gitea
 ENV GITEA_CUSTOM /var/lib/gitea/custom
 ENV GITEA_TEMP /tmp/gitea