[server] Tag the source code for latest ghcr.io Docker image with museum/ghcr

Helps with easily finding changes since last publish.

.github/workflows/server-publish.yml

@@ -38,3 +38,8 @@ jobs:
                   tags: ${{ inputs.commit }}, latest
                   username: ${{ github.actor }}
                   password: ${{ secrets.GITHUB_TOKEN }}
+
+            - name: Tag as museum/ghcr
+              run: |
+                  git tag -f museum/ghcr
+                  git push -f origin museum/ghcr

server/docs/publish.md

@@ -39,3 +39,7 @@ combine both these steps too.
 Once the workflow completes, the resultant image will be available at
 `ghcr.io/ente-io/server`. The image will be tagged by the commit SHA. The latest
 image will also be tagged, well, "latest".
+
+The workflow will also tag the commit it used to build the image with
+`museum/ghcr`. This tag will be overwritten on each publish, and it'll point to
+the code that was used in the most recent publish.