Cache Unchanged Layers (#50)

* Fixup default branch

* Prevent build version from invalidating cached pip installs

* Reuse unchanged image layers when building on CI

.github/workflows/build-deliver.yaml

@@ -19,7 +19,7 @@ jobs:
 
       - name: Publish to GitHub Container Registry
         # TODO: pin to hash
-        uses: elgohr/Publish-Docker-Github-Action@master
+        uses: elgohr/Publish-Docker-Github-Action@main
         with:
           name: ${{ github.repository }}
           registry: ghcr.io
@@ -33,3 +33,5 @@ jobs:
           # create docker image tags to match git tags
           tag_names: true
           buildargs: VERSION_STRING
+          # cache layers that have not changed between builds (eg dependencies)
+          cache: true

Dockerfile

@@ -2,14 +2,14 @@ FROM python:3.7
 
 WORKDIR /opt/app
 
-ARG VERSION_STRING
-ENV VERSION_STRING=$VERSION_STRING
-
 COPY requirements.txt .
 RUN pip install --requirement requirements.txt
 
 COPY . .
 
+ARG VERSION_STRING
+ENV VERSION_STRING=$VERSION_STRING
+
 ENV FLASK_APP=isacc_messaging.app:create_app() \
     PORT=8000