tag & push branch names in detached mode

[dkapanidis]

Currently when 'captain push' is issued in detached mode of a commit with a branch name, captain issues the following error:

[CAPTAIN] Skipping push of app in detached mode

How to reproduce:

• Checkout git with commit-id (detached mode)

git checkout 2225ad4

Git tree example:

* 2225ad4 (HEAD, origin/example, example) Commit example 2
* ef14e6a Commit example 1

Rational:

This is basically the way most CI servers work, they checkout in detached mode, so in order to tag docker image with the branch name we need captain to be able to identify the branches in detached mode and use them.