Replace `docker-test` feature flags with better testing setup

[mkatychev]

Separately, the docker-tests feature should not exist (IMO, like the test feature shouldn't) -- the most elegant way I've seen that navigated is discovering those features in build.rs and injecting a dynamic cfg or ENV that lets the tests do what they need to do. I think the current code was an initial approach at this that should be removed.

Originally posted by @vados-cosmonic in #220 (comment)

[mkatychev]

@vados-cosmonic I'm trying to look for alternatives to the current docker test setup we have. I recall you proposed some alternatives, I was looking around for something similar to mockito and came across this project: https://github.com/gpmcp/registry-testkit.

It's a bit under the radar but it seems to fit our use case quite nicely and removes the need for a docker daemon.

[vados-cosmonic]

Ahhh so I didn't mean to say to not use docker, I just meant that we can do dynamic cfg feature resolution via https://doc.rust-lang.org/cargo/reference/build-scripts.html#rustc-check-cfg and friends -- i.e. we can know if docker is present during pre-test build time (in CI), and enable the tests that use docker completely dynamically.

I was just thinking that docker-tests shouldn't be a "real" feature that we expose.

It's a bit controversial maybe but I am 100% a fan of using testconttainers and actual docker over mocking where reasonable, way closer to the real thing which IMO is important for an E2E test.

[mkatychev]

Let's stick with testcontainers, I'm a bit wary of doing dyanmic cfg resolution without a way of forcing it on explicitly that way the test insn't skipped silently in CI.