Drop dependency on build:gitlab-shell

Set the default user to `git`

.gitlab-ci.yml

@@ -987,7 +987,6 @@ gitlab-shell-libproxyproto:
     - push_tags $GITLAB_SHELL_VERSION${IMAGE_TAG_EXT}
   dependencies:
     - gitlab-shell
-    - build:gitlab-shell
 
 build:git-base:
   <<: *build-job-base

gitlab-shell-libproxyproto/Dockerfile

@@ -12,3 +12,6 @@ RUN cd /tmp && \
     curl --retry 6 -o openssh-sftp-server.deb https://gitlab.com/gitlab-com/gl-infra/openssh-patches/uploads/520bba5376bef780cfd0c7ccefd4338d/openssh-sftp-server_7.9p1-10+deb10u2+gitlab+openssh79+8c55f4e4+1_amd64.deb && \
     dpkg -i openssh-client.deb openssh-server.deb openssh-sftp-server.deb && \
     rm openssh-client.deb openssh-server.deb openssh-sftp-server.deb
+
+ARG GITLAB_USER=git
+USER $GITLAB_USER:$GITLAB_USER