Upgrade podman/docker containers (#850)

* containers: Pull newer versions of containers

Allows topgrade to update a users containers. It will automatically skip
containers which come from the `localhost` repo as these are self-built.
Respects the version number the containers were initially checked out
with in order not to introduce semver-breaking changes.

Works with podman and docker.

* topgrade: Add 'containers' step

* containers: Ignore some errors for docker

This patch is needed to achieve compatibility between docker and podman.
In particular, docker doesn't store/tell the user from which repository
(i.e. `hub.docker.com`, or `registry.fedoraproject.org`) a container
originates. This has the side-effect, that self-built containers cannot
be distinguished from publicly available containers. Therefore this
patch introduces an exception to the error handling when pulling, by
scanning the output of the `docker pull` command. If it finds the
`registry does not exist` substring in the output, it will skip the
container but **NOT** consider the whole update step failed.

* containers: Skip '<none>' containers

that result from either intermediate products of a container build or
when images are dangling.

* steps: containers: simplify error handling

And don't return errors from within the "unknown container registry"
handling, since that would immediately terminate the whole update which
isn't intended.

src/main.rs

@@ -312,6 +312,7 @@ fn run() -> Result<()> {
     runner.execute(Step::Kakoune, "Kakoune", || kakoune::upgrade_kak_plug(&ctx))?;
     runner.execute(Step::Node, "npm", || node::run_npm_upgrade(&ctx))?;
     runner.execute(Step::Pnpm, "pnpm", || node::pnpm_global_update(&ctx))?;
+    runner.execute(Step::Containers, "Containers", || containers::run_containers(&ctx))?;
     runner.execute(Step::Deno, "deno", || node::deno_upgrade(&ctx))?;
     runner.execute(Step::Composer, "composer", || generic::run_composer_update(&ctx))?;
     runner.execute(Step::Krew, "krew", || generic::run_krew_upgrade(run_type))?;