Swap Docker for Podman

[MrHedmad]

Podman is a FOSS alternative to Docker. It has some advantages for non-enterprise users:

• Virtually identical usage with respect to Docker;
• Runs as user by default, generating files owned by the user;
• Perfectly compatible with Docker;
• Can upload to the docker registry, albeit with a bit more configuration.

But:

• Has a worse interface;
• Less known than docker;
• Sometimes requires more configuration than docker;

All considered, it should be better.

[MrHedmad]

I've added the choice of backend from Docker to Podman in #57. The idea is to let the user decide what they prefer. The default is still Docker, however. I think that podman might be better in the long run, but I prefer to test it out some more before making the switch. This issue is resolved, however.