Build(Dockerfile): Compile with LTO, and strip

[ee7]

This PR adds compilation/linking flags that make the runner binary smaller and faster.

[ee7]

I'd add somewhere what the flags do b/c I don't think they're documented.

Good idea - they're definitely under-documented at the moment. It's on the website though, see:

• https://github.com/nim-lang/website/commit/00ff559652a6b44298ad98e38a08703b02eafd21
• https://nim-lang.org/faq.html (Ctrl + f lto)

And they're defined here.

I'll just document it in the commit message for now.

[ynfle]

Those are none of the places that I would have looked for compilation options, and that is the reason that I was confused about what they were. I would have assumed them to be somewhere else like here

[ee7]

I would have assumed them to be somewhere else like here

Yes, I'd expect it there too, given that's where --passC:-flto is mentioned.

It looks like part of the reason that -d:lto is under-documented is that it doesn't work properly on macOS yet. See here.

It might've been better if this PR just did --passC:-flto --passL:-s, but:

• we're only using -d:lto on Linux
• and as mentioned, the source of truth is nim.cfg where we can see that for us it really is just doing

  --passC:-flto --passL:-flto --passL:-s

(--passL:-flto is only needed for pretty ancient versions of gcc).