chore: minor tweaks to travis configuration (#665)

More explicitly skip publishing to DockerHub if this is a PR build. Display
values of `$TRAVIS_TAG` and `$TRAVIS_BRANCH` for more information
when troubleshooting builds (if that's related to publishing).

Release builds from `master` to a `:nightly` tag while (git) tagged builds
will (also) release to `:latest`.

.travis.yml

@@ -10,18 +10,21 @@ services:
 
 before_script:
   # Attempt to pre-cache the previous version of the image, to speed build up
-  - docker pull jsii/superchain:latest || true
+  - docker pull jsii/superchain:nightly || true
 
 script:
   - set -eo pipefail
-  - docker build --pull --build-arg BUILD_TIMESTAMP=$(date -u +'%Y-%m-%dT%H:%M:%SZ') --build-arg COMMIT_ID="${TRAVIS_COMMIT}" -t "jsii/superchain:latest" ./superchain
-  - docker run --rm -it --net=host -v ${PWD}:${PWD} -w ${PWD} jsii/superchain:latest ./build.sh
+  - docker build --pull --build-arg BUILD_TIMESTAMP=$(date -u +'%Y-%m-%dT%H:%M:%SZ') --build-arg COMMIT_ID="${TRAVIS_COMMIT}" -t "jsii/superchain:nightly" ./superchain
+  - docker run --rm -it --net=host -v ${PWD}:${PWD} -w ${PWD} jsii/superchain:nightly ./build.sh
+  - echo "TRAVIS_PULL_REQUEST = ${TRAVIS_PULL_REQUEST:-}"
+  - echo "TRAVIS_TAG          = ${TRAVIS_TAG:-}"
+  - echo "TRAVIS_BRANCH       = ${TRAVIS_BRANCH:-}"
   - >-
-    if [ ! -z "${DOCKER_USERNAME+SET}" ] && [ ! -z "${DOCKER_PASSWORD+SET}" ]; then
+    if [ ${TRAVIS_PULL_REQUEST} = false ] && [ ! -z "${DOCKER_USERNAME+SET}" ] && [ ! -z "${DOCKER_PASSWORD+SET}" ]; then
       echo "${DOCKER_PASSWORD}" | docker login -u "${DOCKER_USERNAME}" --password-stdin
-      if [ ! -z "${TRAVIS_TAG+SET}" ]; then
+      docker push jsii/superchain:nightly
+      if [ "${TRAVIS_TAG:-}" = "${TRAVIS_BRANCH}" ]; then
+        docker tag jsii/superchain:nightly jsii/superchain:latest
         docker push jsii/superchain:latest
-      else
-        docker push "jsii/superchain:${TRAVIS_BRANCH}"
       fi
     fi