Use starship-test docker image for running integration tests (#70)

* Publish docker image used in testing
* Update integration_test to pull the image and build, using the image as its cache
* Publish the built image after successful test runs on master

azure-pipelines.yml

@@ -17,9 +17,7 @@ jobs:
       displayName: Check starship with nightly
 
   # Run the integration tests in a Docker container
-  - job: test_docker
+  - template: ci/azure-test-docker.yml
+    parameters: 
+      name: test_docker
       displayName: Test starship Docker
-    pool:
-      vmImage: ubuntu-16.04
-    steps:
-      - script: ./integration_test

ci/azure-test-docker.yml

@@ -0,0 +1,16 @@
+jobs:
+- job: ${{ parameters.name }}
+  displayName: ${{ parameters.displayName }}
+  pool:
+    vmImage: ubuntu-16.04
+
+  steps:
+  - script: ./integration_test
+    displayName: Run integration test within Docker
+  
+  - script: |
+      docker login -u $(dockerUsername) -p $(dockerPassword)
+      docker push starshipcommand/starship-test
+    # Only push new image if on master and build is passing
+    condition: and(succeeded(), eq(variables['Build.SourceBranch'], 'refs/heads/master'))
+    displayName: Push image to dockerhub

integration_test

@@ -6,8 +6,14 @@ if ! (docker --version); then
   exit 1
 fi
 
+printf 'Pulling latest docker image'
+docker pull starshipcommand/starship-test
+
 printf 'Building test docker image:\n'
-docker build -f tests/Dockerfile --tag starship-test .
+docker build -f tests/Dockerfile \
+  --tag starshipcommand/starship-test \
+  --cache-from starshipcommand/starship-test \
+  .
 
 printf 'Running test suite:\n'
-docker run --rm -t -v $(pwd):/starship starship-test
+docker run --rm -t -v $(pwd):/starship starshipcommand/starship-test