CI - always build docker image, publish it only for a git tag

Seconds release with docker build error - 6ef3519

https://travis-ci.org/EdgedesignCZ/phpqa/jobs/558464512#L2025
  Problem 1
    - Conclusion: don't install sensiolabs/security-checker v6.0.1
    - Conclusion: remove symfony/polyfill-mbstring v1.1.1

https://docs.travis-ci.com/user/deployment/#conditional-releases-with-on
Skipping a deployment with the script provider because this branch is not permitted: fix-docker-build-20190714
https://travis-ci.org/EdgedesignCZ/phpqa/jobs/558468152#L1639

.docker/build-image

@@ -21,8 +21,13 @@ load_docker_variables () {
 
 build_docker_image () {
     set -e
-    docker login --username $DOCKER_USER --password $DOCKER_TOKEN
     time docker build --file $DOCKER_FILE --tag $DOCKER_TAG $APP_DIR
+
+    if [ -z "$TRAVIS_TAG" ]; then
+        echo "Docker image is published only if the current build is for a git tag"
+        exit 0
+    fi
+    docker login --username $DOCKER_USER --password $DOCKER_TOKEN
     time docker push $DOCKER_TAG
 }
 

.travis.yml

@@ -42,7 +42,7 @@ deploy:
     script: .docker/build-image
     skip_cleanup: true
     on:
-      tags: true
+      all_branches: true
       php: 7.0
 # http://blog.wyrihaximus.net/2015/07/composer-cache-on-travis/
 cache: