Build Dagger docs with Dagger

[shykes]

Examples:

• Build the docs site: dagger call --source=. site export --path=./docs-site
• Build the docs server: dagger call --source=. server
• Run docs locally: dagger call --source=. server as-service up
• Run docs for a PR: dagger call --source=https://github.com/dagger/dagger#pull/7382/head server as-service up

[shykes]

To test that this PR works:

dagger -m github.com/dagger/dagger@pull/7396/head call --source=github.com/dagger/dagger#pull/7396/head \
 docs \
 server \
 as-service \
 up

Meta: I have notes on the UX of this command:

1. I reference the same remote git branch twice, but have to use two different syntaxes (# vs @) (is there an issue for this? cc @helderco @jedevc @vito )
2. server as-service up... Container.up() #7399
3. How do I hook up Dagger Cloud so that I can run it on my laptop, and the result is added as a check on this PR? cc @aluzzardi @vito @crjm

[vito]

I reference the same remote git branch twice, but have to use two different syntaxes (# vs @) (is there an issue for this? cc @helderco @jedevc @vito )

This was part of the refs v2 idea, unifying module ref syntax with git ref syntax. Not sure if there's a path to that from @grouville's current work on VCS-independent refs. Maybe?

How do I hook up Dagger Cloud so that I can run it on my laptop, and the result is added as a check on this PR? cc @aluzzardi @vito @crjm

This was disabled for now because it seemed too unusual, wanted to give it more thought. Now the checks will be only added if it's CI=true. I wonder if you could fool it with env CI=true dagger call ...?