workflows: docker: change auth user, and use docker/login-action

This should make automatic updates work again. docker/login-action
allows for authentication with a token instead of password.

Inspiration taken from workflow in github.com/termux/termux-docker.

.github/workflows/docker_image.yml

@@ -23,10 +23,13 @@ jobs:
       run: |
         cd ./scripts
         docker build --tag termux/package-builder:latest .
-    - name: Push to hub.docker.com
-      env:
-        DOCKER_LOGIN: xeffyr
-        DOCKER_PASS: ${{ secrets.DOCKER_HUB_PASSWORD }}
+    - name: Login to Docker Hub
+      if: github.ref == 'refs/heads/master' && github.event_name != 'pull_request' && github.repository == 'termux/termux-packages'
+      uses: docker/login-action@v1
+      with:
+        username: grimler
+        password: ${{ secrets.DOCKER_TOKEN }}
+    - name: Push to Docker Hub
+      if: github.ref == 'refs/heads/master' && github.event_name != 'pull_request' && github.repository == 'termux/termux-packages'
       run: |
-        docker login --username "${DOCKER_LOGIN}" --password "${DOCKER_PASS}"
         docker push termux/package-builder:latest