Use Dependabot to check that the base image is up to date

.github/dependabot.yml

@@ -0,0 +1,6 @@
+version: 2
+updates:
+  - package-ecosystem: docker
+    directory: "/"
+    schedule:
+      interval: weekly

.github/workflows/push.yml

@@ -0,0 +1,59 @@
+name: Push
+
+on: [push, workflow_dispatch]
+
+jobs:
+  build:
+    name: Build and test Docker Image
+    runs-on: ubuntu-20.04
+    timeout-minutes: 10
+
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v1
+
+    - name: Cache Docker Layers
+      uses: satackey/[email protected]
+      continue-on-error: true
+
+    - name: Login to Docker Hub
+      uses: azure/docker-login@v1
+      continue-on-error: true
+      with:
+        username: ${{ secrets.DOCKER_USERNAME }}
+        password: ${{ secrets.DOCKER_PASSWORD }}
+
+    - name: Build Docker Image
+      run: docker build --tag axiom/docker-erddap:latest .
+
+    - name: Run Docker Image in Background
+      run: docker run -d -p 8080:8080 axiom/docker-erddap:latest
+
+    - name: Check that ERDDAP Docker Image will return a 200
+      uses: ifaxity/wait-on-action@v1
+      timeout-minutes: 1
+      with:
+        resource: http://localhost:8080/erddap/index.html
+
+  push:
+    name: Push latest image to Docker Hub
+    runs-on: ubuntu-20.04
+    timeout-minutes: 10
+    needs: build
+    if: (github.ref == 'refs/heads/master' || github.ref == 'refs/heads/main') && github.repository == 'axiom-data-science/docker-erddap'
+
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v1
+
+    - name: Cache Docker Layers
+      uses: satackey/[email protected]
+
+    - name: Login to Docker Hub
+      uses: azure/docker-login@v1
+      with:
+        username: ${{ secrets.DOCKER_USERNAME }}
+        password: ${{ secrets.DOCKER_PASSWORD }}
+
+    - name: Push latest image to Docker Hub if on master or main branch of Axiom's repo
+      run: docker push axiom/docker-erddap:latest

README.md

@@ -17,6 +17,10 @@ This repository will **not** back-port changes from the upstream image to existi
 (for example - SHA512 password hashes) you will have to wait for the next ERDDAP release which will be built with the newest upstream image.
 You can also build this image yourself.
 
+Use `latest` image with caution as it follows the upstream image, and is not as thoroughly tested as tagged images.
+
+[Dependabot](https://docs.github.com/en/free-pro-team@latest/github/administering-a-repository/keeping-your-dependencies-updated-automatically) is used to automatically make PRs to update the upstream image ([`.github/dependabot.yml`](.github/dependabot.yml)).
+
 ## Quickstart
 
 ```bash