Set effective version when building images

[timebertt]

How to categorize this PR?

/area dev-productivity
/kind enhancement
/priority normal

What this PR does / why we need it:
With this PR, docker images built by make docker-images are tagged and build with a version including the commit hash (exactly like the images built by the pipeline).
This allows developers to build prerelease images with a correct version inside to evaluate a new release. E.g. the gardenlet uses the pkg/version.gitVersion to report its own version and to the deploy the same gardenlet image version into shooted Seeds.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

Docker images built by `make docker-images` are now tagged and build with the commit hash appended to the version.

[vpnachev]

Codewise looks fine.
I have some concerns about this change.
git rev-parse HEAD is the commit hash of the last commit which might be something already merged in the master branch, but not your local changes. With this said, before building the image, the changes needs to be committed to get their own commit hash. For example, if I am deploying from master and you build and push local image based on same commit from master, I will use your local changes instead of the current master.

[timebertt]

Thanks for your comments, that's a valid concern 👍
I updated the Makefile to append -dirty to the EFFECTIVE_VERSION, if someone is building from a dirty tree.

[timebertt]

Found a way to calculate the -dirty postfix in the Makefile.
@timuthy @rfranzke @vpnachev PTAL :)

[rfranzke]

/lgtm

[timuthy]

Nice, thanks for implementing the "dirty" use-case 😄
/lgtm