fix the merge request dockerhub build.
var CI_COMMIT_BRANCH is not available in a merge request pipleine.
adjusted the pipline to run if development branch with a commit message
starting with Merge branch.
this enables usage of variable CI_COMMIT_BRANCH.
MR !4
When a git tag occurs, the image will also be tagged 'latest' and be
pushed to docker hub.
When a merge request occurs on the development branch, the image
will also be tagged 'dev' and be pushed to dockerhub.
MR !2