specify a tag, branch or commit during docker prod build

[yancyribbens]

Add build-arg to docker for choosing which branch, tag or commit to run during build time.

[guggero]

The code change is definitely useful and we should have reviewed this a long time ago.

The examples might be a bit confusing as they are, I added some suggestions on how to improve them.

[guggero]

LGTM 👍

[carlaKC]

Awesome change 😻 I'm going to get some good use out of this one.
Looks good to me, and works locally.

Tiny nit on the commit name, I think it makes sense to prefix it with dockerfile: since that's where the change is (and the docs change is related to the docker file)

[yancyribbens]

@carlaKC I see lots of example commits with docker. maybe that would be preferred over dockerfile?

[guggero]

Yes, docker: is fine too, thanks!

[carlaKC]

@carlaKC I see lots of example commits with docker. maybe that would be preferred over dockerfile?

Sorry I missed this, too many git emails 🙈 Thanks for the PR!
docker is good as Oli says.