CI: try brew cask for docker install

Signed-off-by: Stephen Gutekanst <stephen@sourcegraph.com>

Author: Stephen Gutekanst

.github/workflows/ci.yml

@@ -42,7 +42,7 @@ jobs:
       - name: Publish Docker image
         if: success() && github.ref == 'refs/heads/main' && github.event_Name == 'push' && github.repository == 'sourcegraph/doctree'
         run: |
-          brew install docker
+          brew install --cask docker
           echo "$DOCKER_PASSWORD" | docker login -u="$DOCKER_USERNAME" --password-stdin
           task build-image
           docker tag "sourcegraph/doctree:dev" "sourcegraph/doctree:$RELEASE_COMMIT"