This change is done to avoid conflicts when building docker image in PRs (e.g. PR tag a
7.13.0-SNAPSHOT and dev does the same at the same time)

Add a new option `-t` on the `build.sh`. It takes in input a tag name
and it overrides the default tag (`bonitasoft/...:7.x.y-SNAPSHOT`)

This simplifies the build and push of image on `quay` (it avoid using an
intermediate tag)