Older versions of git-annex don't support `git annex lock --force`, so peform
a version check and call without --force on these.
The EL6 Docker image is changed to one that contains locales, in order for
git-annex to run successfully.
Add URL of upstream tito repo to HACKING, then
symlink HACKING as README.md in each Dockerfile dir.
Why?
If a README.md exists in same dir as Dockerfile (incl. symlink),
and it's set up as a
[trusted build](https://index.docker.io/help/docs/#trustedbuilds),
then index.docker.io automatically shows the readme on the info
page of the trusted build.
