Revert "ci: prevent running multiple Docker builds at once."

This reverts commit 9d62f71cd9.

This caused a problem where if you pushed multiple branches or tags at
once, for example to the betabooru and production branches, then the
Docker image would only get built for one branch. This led to deploys
not fetching the latest image.

.github/workflows/docker-build.yaml

@@ -26,8 +26,6 @@ jobs:
   docker-build:
     runs-on: ubuntu-latest
 
-    concurrency: docker-build
-
     steps:
       # https://github.com/docker/setup-buildx-action
       - name: Set up Docker Buildx