Documentation generation without internet connectivity

[cyphar]

Description

When trying to build the docker cli man pages, the scripts for building the man pages (since v23.0.0) require an internet connection because the handful of packages needed to generate the man pages are not vendored and thus need to be pulled at build time. The same applies for the yaml and markdown docs.

This is not possible in the build environment of some distributions (at the very least, openSUSE doesn't permit internet access to build machines but I suspect this is true for a fair number of distributions). The net result is that we cannot release any version of Docker >=23.0.0 at the moment because we cannot package the man pages for Docker.

I've spent a while trying to figure out a way to work around this in various ways, but it seems the simplest solution would be to back to vendoring the packages. This change was added in #3381, but it seems like the purpose of that PR was to simply add a way of building the documentation during CI to validate the build works. The switch to requiring internet access for the build doesn't seem necessary to me and there was no discussion in the PR about this change (indicating that the downstream impact of this change wasn't noticed at the time).

Would you accept a PR that switches back to vendoring the packages needed for documentation generation, removing the need for internet access?

[sam-thibault]

/cc @crazy-max any thoughts on this?