Fix e2e instructions and add a Docker runner for them

[snormore]

Noticed a couple things that were off in the e2e instructions, and also wanted an isolated way to run this in a Docker image.

[timoreimann]

A few gotchas I spotted during testing. I like the idea to containerize the end-to-end tests a lot, great idea. 👏

[snormore]

Comments addressed and should be 👌 for another round of 👀 whenever you get a chance. 🙏

[timoreimann]

Left one more comment about the cleanup script and a few more about public/private key handling. But please read on before addressing my comments.

I've been thinking more about how--or rather if--we should handle keys, especially creating them. My latest and greatest thoughts are that we should leave this entirely to the user. I came to this conclusion tracking backwards, here's my train of thought:

1. Expecting keys to be located under e2e/ (or any version controlled-directory, for that matter) seems worrisome since it's very easy to accidentally commit private keys. While we git-ignore e2e/id_rsa*, we cannot assume that everybody uses that. Even if we were to git-ignore a specific directory, there's still some risk that users put their keys elsewhere and commit them unintentionally.
2. The directory where keys should be located in is $HOME/.ssh. That's the safest place to, and it's it's also what SSH tooling is tailored for (e.g., to ensure proper file permissions are set). Some users may even want their CCM end-to-end key to be located there.
3. We definitely should not create keys in $HOME/.ssh from our scripts.

Consequently, I think we should leave it to the user to set up their keys accordingly.

What we could theoretically do is check if the fingerprint is found in the account (via doctl) and bail out if it isn't. That requires being able to read the key from inside the container, which probably means mounting in $HOME/.ssh again as read-only.

Thoughts on that?

[timoreimann]

Two minor things we need to revert to account for the change of course in regards to SSH key usage.

Approving already to save one roundtrip.

Thanks a lot @snormore 👏