[WIP] Add support for Red Hat UBI

[HarryMichal]

Adding support for [Red Hat UBI] (Universal Base Image) is a requirement for Toolbox to be included in RHEL (and in the process replacing https://github.com/coreos/toolbox).

Adding support for a new image requires implementing an internal "store" of supported images. To be frank, this is something I dreaded quite a bit for some time already :). This implements a very basic image store that allows matching based on a os-release:ID (e.g., fedora is Fedora, rhel is RHEL - in case of Toolbox UBI,..). This matching is available through a new option --distro | -d. The existing --release | -r option complements the new option.

The updated workflow should look a bit like this:

• Scenario 1 - host machine is Fedora 33

$ toolbox create              <- creates a Fedora 33-based container
$ toolbox create -r 31        <- creates a Fedora 31-based container
$ toolbox create -d rhel -r 7 <- creates a RHEL 7-based container

• Scenario 2 - host machine is RHEL 8

$ toolbox create           <- creates a RHEL 8-based container
$ toolbox create -r 7      <- creates a RHEL 7-based container
$ toolbox create -d fedora <- creates a Fedora 32-based container (default version of Fedora is used)

• Scenario 3 - host machine is Arch Linux

$ toolbox create <- creates a Fedora 32-based container

On distributions without a matching supported image will Fedora stay as the default.

While this adds only support for UBI, technically speaking, it should be possible to add support for even other images. Another matter is who is going to maintain the images and where they will be stored. Before these questions are answered, we'll continue offering only Fedora and UBI images (but you are always free to use an image of your own making!).

[softwarefactory-project-zuul]

Merge Failed.

This change or one of its cross-repo dependencies was unable to be automatically merged with the current state of its repository. Please rebase the change and upload a new patchset.

[debarshiray]

Thank you for working on this!

Let's see if we can split the commits a bit more. eg., one where ResolveContainerAndImageNames gets replaced by the new image backend, followed by one where --distro gets added and so on.

Some quick comments:

[debarshiray]

The latest part makes me a bit nervous. :)

I did toolbox create on my F33 host, and got a F34 container named fedora-toolbox-latest. Another point of confusion would be when the host gets upgraded, or the value of latest changes over time.

[debarshiray]

I am wondering if we should call it Distro, instead of Image. Then we could include a function pointer to parse the release string in a distro-specific way.

[debarshiray]

I split up the commits, and fixed up some of the above issues in #667

Most importantly, I had to skip some parts of the new container and image tracking backend to side-step some issues that I couldn't figure out on my own. eg., the one caused by the use of latest tags. Let's keep this pull request open to work through those.

[travier]

We've made progress in this front in #1019. See the top comment for links to community supported images.