add docker push instructions

.travis.yml

@@ -5,9 +5,13 @@ services:
 
 before_install:
   - docker build -t hbclab/accel-bids -f Dockerfile_exec .
-  - echo "$DOCKER_PASS" | docker login -u "$DOCKER_USERNAME" --password-stdin
-  - docker push hbclab/accel-bids:latest
 
 script:
   - docker run --entrypoint /bin/bash hbclab/accel-bids -c ". activate accel; pytest /home/coder/projects"
 
+deploy:
+  provider: script
+  script: bash docker_push.sh
+  on:
+    branch: master
+    tags: true

docker_push.sh

@@ -0,0 +1,9 @@
+echo "$DOCKER_PASS" | docker login --username "$DOCKER_USERNAME" --password-stdin
+# if tag is defined
+if [[ ! -z "$TRAVIS_TAG"]]; then
+    docker push hbclab/accel-bids:$TRAVIS_TAG
+# if the build is on the master branch
+elif [[ "$TRAVIS_BRANCH" == "master" ]]; then
+    docker push hbclab/accel-bids:unstable
+else
+    echo "This build is not on the master branch or a tagged release"