Extract deployment logic to its own script

And use Travis deployment feature to run it (which allows conditional execution):

Author: michaelbaudino

.travis.yml

@@ -10,6 +10,12 @@ before_script:
 
 script: ./build.sh
 
+deploy:
+  provider: script
+  script: ./publish.sh
+  on:
+    branch: master
+
 env:
   global:
     - DOCKER_BUILDKIT=1

build.sh

@@ -1,8 +1,11 @@
 #!/bin/bash
 
-set -ev
+if [ -z "${RUBY_VERSION}" ]; then
+  echo "Error: \$RUBY_VERSION is unset. Exiting."
+  exit 64
+fi
 
-### Build
+set -ev
 
 docker build . \
   --build-arg BASE_IMAGE_TAG=${RUBY_VERSION} \
@@ -12,16 +15,3 @@ docker build . \
       echo "--tag alpinelab/ruby-dev:${alias}"
     done
   )
-
-### Publish
-
-if [[ ${TRAVIS_BRANCH} = "master" && -z ${TRAVIS_PULL_REQUEST_BRANCH} ]]; then
-  # Publish with base image name
-  docker push alpinelab/ruby-dev:${RUBY_VERSION}
-  # Publish as aliases
-  for alias in ${ALIAS//,/ }; do
-    docker push alpinelab/ruby-dev:${alias}
-  done
-else
-  echo "This is not a commit on \`master\`: skip publishing.";
-fi

publish.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+
+if [ -z "${RUBY_VERSION}" ]; then
+  echo "Error: \$RUBY_VERSION is unset. Exiting."
+  exit 64
+fi
+
+set -ev
+
+# Publish default name
+docker push alpinelab/ruby-dev:${RUBY_VERSION}
+
+# Publish aliases
+for alias in ${ALIAS//,/ }; do
+  docker push alpinelab/ruby-dev:${alias}
+done