Create dependabot.yml

[jauderho]

Add dependabot support to keep GitHub Actions and Docker up to date.

I will submit a separate PR to convert workflows to use commit hashes.

[rui314]

It's not clear to me what this prevents. We are using Docker for testing and creating release binaries, but that's not something we are continue running on a server. So is GitHub Actions -- we are using it only for running tests. What is a possible threat and how useful it is to protect from it?

[aloisklink]

We are using Docker for testing and creating release binaries, but that's not something we are continue running on a server.

I'm pretty sure @dependabot can't yet update Docker dependencies in a GitHub Action workflow, so this change is currently pointless.

So is GitHub Actions -- we are using it only for running tests. What is a possible threat and how useful it is to protect from it?

The main issue is that

mold/.github/workflows/ci.yml

Lines 24 to 25 in b78f237

	- name: ccache
	uses: hendrikmuhs/ccache-action@v1

references the v1 tag of https://github.com/hendrikmuhs/ccache-action

If https://github.com/hendrikmuhs/ccache-action updates this tag to contain a bug, this could break the CI workflows, and it might be difficult to figure out that the bug is caused by hendrikmuhs/ccache-action.

Using a commit hash (like hendrikmuhs/ccache-action@53e53642769c4b03a7881a7283314d1f364134ec) is the best practice when you don't trust the author to break their action. See https://docs.github.com/en/actions/security-guides/security-hardening-for-github-actions#using-third-party-actions. You can then use @dependabot to automatically update this hashes to the latest version. But I think this is probably overkill to add to hendrikmuhs/ccache-action (and will just add a lot of noise to the git logs), since in the unlikely event that an hendrikmuhs/ccache-action update breaks something, it will probably just break tests. Every other action is owned by @github or @rui314, so it's also not needed.

But, it might be nice to add @dependabot just so that you don't have to manually update all of the actions/... the next time they have a breaking change. Pretty much all GitHub Actions actions used to run on Node.JS v12, but did a major release when they upgraded to Node.JS v16 (e.g. actions/checkout@v3). And with Node.JS 16 is become end-of-life earlier than normal on 2023-09-11, enabling @dependabot means we won't need to manually bump all of these versions.

[aloisklink]

This file is invalid YAML, since the indentation is wrong, see:

Error : bad indentation of a sequence entry at line 13, column 4:
       - package-ecosystem: "docker" #  ... 
       ^
Line : undefined  undefined

[jauderho]

Good catch. Indentation fixed.

[jauderho]

@aloisklink The GH action will update common/Dockerfile‎ once I update dependabot.yml in my branch.

I'm guessing that the Dockerfile in / got moved.

[rui314]

I'm not still convinced that this is necessary. It seems like Dependabot is for web apps or something like that and may not suit well for us. We could still deploy it anyway, but the value of doing it doesn't seem that much.