Use path context for docker CI action

Author: jacob-rosenthal

.github/workflows/publish-to-docker-hub.yml

@@ -8,6 +8,8 @@ jobs:
   docker:
     runs-on: ubuntu-latest
     steps:
+      - name: Checkout
+        uses: actions/checkout@v2
       - name: Set version number variable
         run: |
           VER=$(cat pathml/_version.py | grep -o  '"[^"]\+"' | tr '"' ' ')
@@ -25,6 +27,7 @@ jobs:
         id: docker_build
         uses: docker/build-push-action@v2.7.0
         with:
+          context: .
           push: true
           tags: |
             pathml/pathml:latest