update default branch to master

- fix URL for CI badge in README
- add actions workflow to push 'main' branch to 'latest' docker image

.github/workflows/docker-latest.yml

@@ -0,0 +1,18 @@
+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-tags.yml

@@ -2,7 +2,7 @@ on:
   push:
     tags:
     - '*'
-name: docker-publish
+name: docker-publish-tags
 
 jobs:
   docker: