Relates to a82fff6377/docs/packages.md (proxies)
> (..) the first four of these are the standard built-in build-arg options
> available for `docker build`
> (..) The last, `all_proxy`, is a standard var used for socks proxying. Since
> it is not built into `docker build`, if you want to use it, you will need to
> add the following line to the dockerfile:
>
> ARG all_proxy
Given the we support all other commonly known proxy env-vars by default, it makes
sense to add this one as well.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
5a5f8564ba