Fix usage of out-of-date Gitaly images

Our CI jobs and docker-compose pull in the "latest" tag of Gitaly. As it
turns out though, "latest" is pointing to Gitaly v13.3.0-rc5, which is
definitely not the latest versionat this point in time. This is because
CNG was converted to not use the "latest" tag anymore, but instead to
use a tag called "master" in gitlab-org/build/CNG!519.

Fix this by using the new "master" tag instead.

.gitlab-ci.yml

@@ -45,7 +45,7 @@ default:
     - go version
     - which go
   services:
-    - name: registry.gitlab.com/gitlab-org/build/cng/gitaly:latest
+    - name: registry.gitlab.com/gitlab-org/build/cng/gitaly:master
       # Disable the hooks so we don't have to stub the GitLab API
       command: ["bash", "-c", "mkdir -p /home/git/repositories && rm -rf /srv/gitlab-shell/hooks/* && exec /usr/bin/env GITALY_TESTING_NO_GIT_HOOKS=1 /scripts/process-wrapper"]
       alias: gitaly

docker-compose.yml

@@ -3,6 +3,6 @@ services:
   gitaly:
     environment:
       - GITALY_TESTING_NO_GIT_HOOKS=1
-    image: registry.gitlab.com/gitlab-org/build/cng/gitaly:latest
+    image: registry.gitlab.com/gitlab-org/build/cng/gitaly:master
     ports:
       - '8075:8075'