WIP: Podman backend

[manics]

I had a go at implementing a Podman backend (Docker has mysteriously broken itself on my Fedora install)

Main issues:

• There may be some bugs with hard-links (affects conda, needs to be fixed in the upstream podman project, but coming up with a minimal example for a bug report is tricky)
• Travis VM is too old to run podman in user-mode (requires the fuse-overlayfs driver). The options are to use VFS (works but extremely inefficient) or run as root. On a modern OS such as Fedora 30 running Podman as a normal user with fuse-overlayfs should work.
• The Podman Python client library uses the varlink interface which adds unnecessary complication since one of the aims of Podman is to run with minimal privileges and no daemon so I've written a wrapper library that executes podman
• cache_from isn't implemented by podman

Todo:

• Implement push_image
• Implement volumes for podman run
• Get rid of print/debug statements
• Failing test: REPO_TYPE=stencila-py ENGINE=podman
• Check whether Repo2Docker.find_image works
• Refactor behind an Engine class

Closes #682

[betatim]

Neat! Have you thought about having a Engine class with an interface behind which we hide each engine? Was thinking that for two engines we can have if statements to select what to do but for more it would be nice to have something like the contents providers where repo2docker doesn't have to care which one is doing the work.

[manics]

Definitely! This started as a "will it even work" mini project, now that it looks feasible I'll work on the design.

[yuvipanda]

This is awesome and I love it! <3

[manics]

@yuvipanda Thanks! I haven't had much time to work on this recently, but I've upgraded to Fedora 31 which uses cgroups2 which Docker doesn't (yet) support... so I've only got Podman now.

[yuvipanda]

Ah, another casuality in the Leonard Pottering / Solomon Hykes war! Hope podman treats you well!

[manics]

This has been replaced by:

• [MRG] Define an interface for Container engines #848 which adds a ContainerEngine interface
• https://github.com/manics/repo2docker-podman which implements the ContainerEngine interface using podman

[meeseeksmachine]

This pull request has been mentioned on Jupyter Community Forum. There might be relevant details there:

https://discourse.jupyter.org/t/binderhub-for-hpc/143/18