actions/setup-ssh: Direct access to runner machines over Tailscale

[tsibley]

Useful for debugging issues with GitHub Actions workflows, particularly in cases when similar commands work on your machine but not the runner machines.

See also the README.

Related issue(s)

• https://github.com/nextstrain/private/pull/72

Testing

• Manual testing
• Checks pass

[tsibley]

Note that I haven't (yet) defined a TAILSCALE_AUTH_KEY secret at any level (repo or org), because some internal discussion is probably warranted about the account to use and how to best share access to the team before I set things up. However, I have used single-use ephemeral auth keys for my personal Tailscale account to test https://github.com/nextstrain/private/pull/72.

[Update: See also the notes about this I dropped in Slack.]

[tsibley]

To personally test this, you'll need:

• At least one SSH key associated with your GitHub user
• A personal Tailscale account

Steps:

1. Generate a single-use and ephemeral Tailscale auth key
   
2. Launch the debugging-runner workflow in our nextstrain/private repo, providing the auth key as an input.
3. Follow logs from the run and look for output like ssh runner@100.x.y.z
4. Use it to login! Tailscale must be installed and running on your computer.
5. When you're done, run touch ~/continue on the runner machine to allow the workflow to finish. (Or click the "Cancel workflow" button on the web.)

[tsibley]

I was using this again on Friday to try out commands on a runner while adding the Singularity runtime to Nextstrain CLI's CI. Since it's working well for me and there's zero consequence right now if it doesn't, I'm going to merge this. That will let me address https://github.com/nextstrain/private/pull/72#discussion_r1052611618.

We can address providing an org-level Tailscale auth key (per #34 (comment)) separately.

[victorlin]

Thanks for setting this up! Works for me. Made a few changes over at https://github.com/nextstrain/private/compare/93cfadce8e28580c868576d6e3d980b9c99c09fe...4a5756f450d0a56e26ea4cde6981d4bd3df9a83b based on my experience.