fix publishing release image closes #2

adds logic of publishing docker image with tag based on current branch or tag: if its main branch -> image tag=latest if its tagged branch -> image tag=release

The image tag name is passed as project property to build.gradle

also fixes issue with checking out git repository inside gitlab-runner

Notes: It is actually possible to build same docker image with multiple tags but the 2.5.2 version of springboot, we are using, doesn't support it