Attempt, and document, and possibly fix setting up Deploy against gitlab.com

[squaremo]

We've a report through weave-community#flux that using a repo at gitlab.com breaks the Deploy config.

Possible difficulties:

• it's not obvious where to install the deploy key (though gitlab does have SSH keys in the same way github does, see screenshot)

On the other hand, it's reporting that it can clone the repo, it just can't write to it.

• possible problem with checking write permissions for an empty repo (the OP said "I should maybe add that the repo is empty, or only a readme.")

• known_hosts per Need ability to add to known_hosts fluxcd/flux#724, though we add it to the flux image

• a bug which means that it works in the daemon, but reports that it hasn't worked via the API

There's evidence that it can in fact write to the repo:

david [2 hours ago]
git clone [email protected]:davidkarlsen/weave-cloud.git
Cloning into 'weave-cloud'...
remote: Counting objects: 6, done.
remote: Compressing objects: 100% (2/2), done.
remote: Total 6 (delta 0), reused 0 (delta 0)
Receiving objects: 100% (6/6), done.
/tmp # cd we*
/tmp/weave-cloud # git tag test
/tmp/weave-cloud # git push --tags
Total 0 (delta 0), reused 0 (delta 0)
To gitlab.com:davidkarlsen/weave-cloud.git
 * [new tag]         test -> test
/tmp/weave-cloud #