ci: Move collab to Dockerfile-collab (#18515)

This makes it possible to have multiple Dockerfiles, each with their own
`.dockerignore`. Previously any docker builds would always include
anything inside `.dockerignore`. I believe this feature may require
`export DOCKER_BUILDKIT=1` but we use that in CI already.

.github/workflows/deploy_collab.yml

@@ -76,7 +76,11 @@ jobs:
           clean: false
 
       - name: Build docker image
-        run: docker build . --build-arg GITHUB_SHA=$GITHUB_SHA --tag registry.digitalocean.com/zed/collab:$GITHUB_SHA
+        run: |
+          docker build -f Dockerfile-collab \
+            --build-arg GITHUB_SHA=$GITHUB_SHA \
+            --tag registry.digitalocean.com/zed/collab:$GITHUB_SHA \
+            .
 
       - name: Publish docker image
         run: docker push registry.digitalocean.com/zed/collab:${GITHUB_SHA}