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.yml

@@ -1,18 +0,0 @@
-on:
-  push:
-    tags:
-    - '*'
-name: docker-publish
-
-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
-        tag_with_ref: true