Introduce coding standards workflow

[greg0ire]

This is a copy of the coding standards workflow I introduced in doctrine/annotations, with a few changes:

• consistent use of coding standards, and in case we add more jobs
• use of actions/cache v2 (I forgot to use it in doctrine/annotations
• I renamed the last step, which still wrongly references phpcbf

I'm deliberately naming the file with a very narrow scope, so that it's easy to enable or disable workflows based on the repository we are using.

[greg0ire]

20.04 is documented as "preview only", apparently latest still resolves to 18.04 if we believe these docs: https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#jobsjob_idruns-on

Tell me if I should use 18.04 here of if you think it's fine

[ostrolucky]

I preferred latest so we don't have to keep updating this string... I don't care about actual version used.

[morozov]

I'd try to use the newest one that works. This way, we won't have to update the workflow too soon.

[greg0ire]

I preferred latest so we don't have to keep updating this string

If we use templates the need to update will only be on this repo, so no longer such a big deal.

[greg0ire]

Huh… turns out this whole sharing feature was not what I thought it was… looks like it is just here to help you bootstrap the action, but you still have to commit a file… and I'm not sure about what happens when you make changes upstream.
Better than nothing I suppose.

[morozov]

I see some point in why that is. The action depends on certain APIs but doesn't own the dependency constraint, so it may be challenging to maintain it once it's shared between projects. The question is, what is the purpose of this shared workflow once all projects are on board? I.e. is there a way to synchronize them with the updates upstream? Otherwise, it may just stale overtime.

[greg0ire]

I'm hoping to find that out by getting doctrine/annotations#357 merged and then seeing if there are any changes in the UI above. But maybe somebody well-versed in this already knows. @localheinz maybe?

[localheinz]

@greg0ire

As far as I understood, these templates are really only templates to simplify creation of workflows for repositories within an organization. As such, they do not propagate as actual workflows to repositories within an organization.

[greg0ire]

Yes, what I'm hoping is that once you create one, it does not propose to "setup the workflow", but rather to synchronize it (by making another commit).