actions: combine docker workflows

.github/workflows/docker-latest.yml

@@ -1,18 +0,0 @@
-on:
-  push:
-    branches:
-    - 'main'
-name: docker-publish-latest
-
-jobs:
-  docker:
-    runs-on: ubuntu-latest
-
-    steps:
-    - uses: actions/checkout@v2
-    - uses: docker/build-push-action@v1
-      with:
-        username: ${{ secrets.DOCKER_USERNAME }}
-        password: ${{ secrets.DOCKER_PASSWORD }}
-        repository: willnorris/imageproxy
-        tags: latest

.github/workflows/docker.yml

@@ -1,5 +1,7 @@
 on:
   push:
+    branches:
+    - 'main'
     tags:
     - '*'
 name: docker-publish-tags