Offer obfs4 docker image for additional architectures

A bridge operator asked us to cross-compile our docker image for arm64. This sounds like an easy-ish fix that would make the lifes of our bridge operators easier. Let's figure out what it takes to support ARM and potentially other architectures.

Trac:
Parent Ticket: legacy/trac#31281 (moved)

added component::circumvention in Legacy / Trac docker in Legacy / Trac parent::31281 in Legacy / Trac points::1 in Legacy / Trac priority::medium in Legacy / Trac severity::normal in Legacy / Trac sponsor::30-can in Legacy / Trac status::new in Legacy / Trac type::enhancement in Legacy / Trac labels

On Linux, cross-architecture builds for docker are currently still an experimental feature that's provided by the "buildx" module. I'll briefly summarise how I got this module to run.

On my Debian buster, I installed the docker-ce-cli package and linked the module /usr/libexec/docker/cli-plugins/docker-buildx to ~/.docker/cli-plugins/docker-buildx. I then enabled experimental features by writing the following to the file /etc/docker/daemon.json:

{                                                                              
    "experimental": true                                                         
}

Next, I restarted the docker service by running sudo service docker restart. Finally, I had to export the environment variable DOCKER_CLI_EXPERIMENTAL=enabled. Once all of that was done, I was able to use experimental features:

$ docker version -f '{{.Server.Experimental}}'
true

...and in particular, the buildx plugin. I stumbled upon a useful blog post that provides an overview of this process.

Trac:
Keywords: s30-o24a2 deleted, N/A added

changed time estimate to 8h

mentioned in issue legacy/trac#31281 (moved)

moved from legacy/trac#33088 (moved)

added Feature label and removed 1 deleted label

removed 1 deleted label