Provide Cloudflare (or other) preview on forked PRs

[rfay]

In today's contributor training we noted that Cloudflare previews aren't available on forked PRs. @chadcarlson and @gilzow mentioned that they have a technique that was used for forked PRs on Platform.sh docs.

[bmartinez287]

https://developers.cloudflare.com/pages/platform/known-issues
"Commits/PRs from forked repositories will not create a preview. Support for this will come in the future."
I wonder what worked around they used for it.

[gilzow]

It's a technique, but I'm not sure I would recommend it.

Bulk of the work is in this workflow: https://github.com/platformsh/platformsh-docs/blob/main/.github/workflows/build-from-fork.yaml

Essentially, for PRs from forks, we create a duplicate branch in the base repository from the fork's branch once the PR has been labeled as build-fork. We then push the duplicate to platform.sh and activate it as an environment. from then on, pushes to the PR are forced-pushed onto the cloned copy of the branch.

Once the PR is accepted, we have to handle deactivation and deletion of the cloned branch : https://github.com/platformsh/platformsh-docs/blob/main/.github/workflows/manage-environment.yaml#L72

It works but not without issues. Since we need to do things like add labels and comments, we have to run these workflows using the pull_request_target event instead of the pull_request event. This is potentially very dangerous. I'm not exactly happy with this setup as we have it but it's the best given our constraints.