deploy: Add /repo endpoint for repository-only setup

[vvoland]

Add a new /repo endpoint (get.docker.com/repo) that configures Docker's package repositories without installing Docker packages.

This is the same install.sh just with REPO_ONLY defaulting to 1, equivalent to running install.sh --setup-repo.

This allows to do:

curl -s https://get.docker.com/repo | sudo sh -

instead of:

curl -s https://get.docker.com | sudo sh -s -- --setup-repo
# or
curl -s https://get.docker.com | sudo REPO_ONLY=1 sh -

For the last one, it's especially tricky because there's a difference between:

# good
sudo REPO_ONLY=1 sh -
and
# bad, sudo would need -E to preserve env
REPO_ONLY=1 sudo sh -

[vvoland]

@thaJeztah WDYT?

[thaJeztah]

So is this to update the install script to get another option, or a completely separate script? (I see /repo as "endpoint" mentioned); not sure if we want a separate script for it, or just roll it up in the existing one?

[thaJeztah]

Oh, right; we already had sudo sh install-docker.sh --setup-repo - wouldn't that be enough?

[vvoland]

Yes we already have it, but it's a bit awkward to use when you pipe from curl.
See the description of the PR.