Add glab in release-cli docker image

We are adding the glab executable in the release-cli image so that
all new release-cli:latest image fetch will include glab
(#194).

This will help our long-term goal of migrating from release-cli to glab
(&15437).

Also, we will be able to continue working on
gitlab#442785.

Dockerfile

@@ -6,7 +6,10 @@ WORKDIR /release-cli
 
 RUN make build
 
+FROM registry.gitlab.com/gitlab-org/cli:v1.47.0 AS glab-source
+
 FROM alpine:3.18
 
 COPY --from=builder /release-cli/bin/release-cli /usr/local/bin/release-cli
+COPY --from=glab-source /usr/bin/glab /usr/local/bin/glab
 COPY --from=builder /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/