workflows/docker: continue to deploy master images

I noticed that `master` images have not been updated since #18912:

    $ docker run --pull=always --rm -it homebrew/brew:master brew --version
    master: Pulling from homebrew/brew
    Digest: sha256:3812ffd9b728ce3d96a2a362ef33bed420d1dc73c7d96c93a8f8d2d4f10e6281
    Status: Image is up to date for homebrew/brew:master
    Homebrew 4.4.11-6-geae8d1b
    Homebrew/homebrew-core (git revision 9610909d254; last commit 2024-12-10)

That is due to images no longer being built on `master` pushes. This
change restores the previous behaviour.

.github/workflows/docker.yml

@@ -2,6 +2,9 @@ name: Docker
 
 on:
   pull_request:
+  push:
+    branches:
+      - master
   merge_group:
   release:
     types:
@@ -74,7 +77,7 @@ jobs:
                 "homebrew/brew:latest"
               )
             fi
-          elif [[ "${GITHUB_EVENT_NAME}" == "merge_group" &&
+          elif [[ "${GITHUB_EVENT_NAME}" == "push" &&
                   "${GITHUB_REF}" == "refs/heads/master" &&
                   "${{ matrix.version }}" == "22.04" ]]; then
             tags+=(