[Enhancement]: Being able to use Ryuk in (CI) environment where Docker "userns-remap" mode is active

Module

Core

Proposal

Hello,

I'm using Testcontainers in an on-prem GitLab CI environment, where the GitLab runner hosts have their Docker server with userns-remap turned on.
I ran into a couple of issues related to Ryuk in this case.
First, from what I read on https://java.testcontainers.org/features/configuration/#customizing-ryuk-resource-reaper, the Ryuk container must be started as a privileged container.
That's the reason why I get the following error when Testcontainers tried launching the Ryuk container:

...
com.github.dockerjava.api.exception.BadRequestException: Status 400: {"message":"privileged mode is incompatible with user namespaces.  You must run the container in the host namespace when running privileged mode"}
...

As a quick hack, I worked around this issue by "monkeypatching" org.testcontainers.utility.RyukContainer and adding:

    cmd
        .getHostConfig()
        ...
        .withUsernsMode("host")  /// <= added

The Ruyk container could then start fine.

But due to the fact the GitLab runner container conforms to this userns-remap mode (and thus not Ruyk as per the above), Testcontainers was not able to connect to Ryuk:

...
WARN --- [containers-ryuk] o.t.utility.RyukResourceReaper : Can not connect to Ryuk at 172.17.0.1:35592

Again, I had to resort to monkeypatching org.testcontainers.utility.RuykResourceReaper to have:

...
// String host = ryukContainer.getHost();
String host = ryukContainer.getCurrentContainerInfo().getNetworkSettings().getIpAddress();
// Integer ryukPort = ryukContainer.getFirstMappedPort();
Integer ryukPort = ryukContainer.getExposedPorts().get(0);
...

My understanding is that this fix is obviously not portable in environments where the Docker host is not local to where Testcontainers is running.
But bar changing my CI environment setup to drop the userns-remap mode, I don't see any other solution to my issue.

What do you think of having "official support" for such a workaround in Testcontainers, so I don't have to monkeypatch Java classes... 🤓
If not via a setting (e.g. ryuk.container.userns_mode=host or something related), maybe via a way to customize the behavior of Testcontainers dynamically?

Thanks!

Review org.testcontainers.utility.RyukContainer and RyukResourceReaper, along with the custom Ryuk configuration documentation. Trace how the container host namespace and connection address are selected, then determine how an official option could support Docker userns-remap without breaking remote Docker environments. Done should include tests covering the supported configuration and connection behavior.