fix(rust): add fallback to Podman sockets for docker_environments

[thiago-carbonera]

Fixes #22936.

Problem

Currently, users utilizing Podman for docker_environments face connection errors because the Rust engine (using the Bollard client) strictly defaults to the standard Docker socket (/var/run/docker.sock). When this socket is not found, the execution fails, forcing users to manually set the DOCKER_HOST environment variable to point to their Podman socket.

Solution

This PR introduces a fallback mechanism when initializing the Docker client in docker.rs:

• It first attempts to connect using standard local defaults.
• If that fails, it proactively checks for the existence of common Podman socket paths:
  Rootless Podman: $XDG_RUNTIME_DIR/podman/podman.sock
  Rootful Podman: /run/podman/podman.sock
• If either socket exists, it establishes the connection using Docker::connect_with_socket.
• If all attempts fail, it correctly bubbles up the original Docker connection error.

This eliminates the need for manual workarounds and provides out-of-the-box support for Podman users.

[sureshjoshi]

Thanks for the PR, but I don't think this is the right approach.

A quick glance at the bollard changelogs shows that they seem to have been improving their Podman support. For example: fussybeaver/bollard#700

I think the fundamental issue is that our Bollard dep is 1.5 years old.

[tdyas]

The latest version of bollard supports looking for podman sockets. Maybe just upgrade bollard to v0.21.0 which claims "first-class Podman support with feature flags" in the release notes?

[tdyas]

Does podman work with docker_environment if you just upgrade the bollard crate?

[thiago-carbonera]

Hi @tdyas and @sureshjoshi! I just bumped the bollard workspace dependency to 0.21.0 locally to test this approach.
It turns out that upgrading from 0.18.1 to 0.21.0 brings a massive amount of API breaking changes from Bollard's side. Things like CreateContainerOptions and Config were heavily refactored, models were moved, and generics were dropped. This results in 30+ compilation errors across process_execution/docker/src/docker.rs.
Since fixing all of these requires a substantial refactor of the Docker engine code, how would you prefer to proceed?

[tdyas]

What version of bollard was the podman support added in? And how much "churn" is there in bollard for that version (versus upgrading all the way to 0.21.x)?

[thiago-carbonera]

Hi @tdyas! I just checked the bollard release notes, and the 'first-class Podman support' was introduced exactly in version 0.21.0. Because it was only added in 0.21.0, there isn't an intermediate version we can upgrade to.
Let me know what i should do.

[tobni]

#23552

I also think @thiago-carbonera you need to consider dialing back how much you allow an LLM to speak for you, if you are. If you're not I apologize in advance.