chore(ci): make TravisCI check for unexpected source changes

.travis.yml

@@ -14,8 +14,13 @@ before_script:
 
 script:
   - set -eo pipefail
+  # Building the Docker image
   - docker build --pull --build-arg BUILD_TIMESTAMP=$(date -u +'%Y-%m-%dT%H:%M:%SZ') --build-arg COMMIT_ID="${TRAVIS_COMMIT}" -t "jsii/superchain:nightly" ./superchain
+  # Building jsii itself within the Docker image
   - docker run --rm -it --net=host -v ${PWD}:${PWD} -w ${PWD} jsii/superchain:nightly ./build.sh
+  # Make sure the build did not change the source tree (git diff-index will return non-zero if that's the case)
+  - git diff-index --exit-code --ignore-space-at-eol --stat HEAD
+  # Publish the image to DockerHub when relevant
   - echo "TRAVIS_PULL_REQUEST = ${TRAVIS_PULL_REQUEST:-}"
   - echo "TRAVIS_TAG          = ${TRAVIS_TAG:-}"
   - echo "TRAVIS_BRANCH       = ${TRAVIS_BRANCH:-}"

buildspec.yaml

@@ -13,6 +13,7 @@ phases:
   post_build:
     commands:
       - "[ -f .BUILD_COMPLETED ] && /bin/bash ./pack.sh"
+      - git diff-index --exit-code --ignore-space-at-eol --stat HEAD
 artifacts:
   files:
     - "**/*"