Make sure we generate a proper docker tag

[krnowak]

I have a branch named like /, which becomes a part of
the tag name. Docker complains about it, because / is not a valid
character in the tag.

This commit replaces all sequences of invalid chars with a single dash
and makes sure that the tag name never exceeds the 127 chars limit.

[alban]

This is similar to #1655

And probably failing for the same reason.

[tomwilkie]

Hi @krnowak; can you rebase this on top of master? It should pass now.

[krnowak]

Updated, lint seems to fail with some typos. For lint fixes please see #1751.

[krnowak]

Updated, so we can be sure that tag names in circleCI are also valid.

[2opremio]

@krnowak / @alban Do we still need this?

[krnowak]

Up to you, really. But now I guess this PR would need to be split in two - one for image-tag to be filed in build-tools repository and one for scope importing the new build-tools version and modifying circle.yml.

My issue was the same as the one @alban had - I had / in my branch name and docker did not like it. I just made extra sure that the tag we generate based on a branch name is always valid (no invalid chars and shorter than 128 chars).

[tomwilkie]

Already an image-tag in build-tools: https://github.com/weaveworks/build-tools/blob/master/image-tag