[Bug]: Unhandled Docker stderr warnings break container inspection during network reachability check on ARM64 macOS

Bug Description

When running vcluster create in Docker driver mode on ARM64 macOS (Apple Silicon), if Docker emits a platform architecture mismatch warning on stderr (e.g. when an amd64 alpine network test image is cached locally), vcluster captures the warning text along with the container ID.

vcluster then passes the multi-line string directly to docker inspect, causing a fatal crash with a confusing error: no such object: WARNING: ... error.

Environment

• vCluster CLI Version: v0.36.1
• OS / Arch: macOS (darwin/arm64)
• Docker Engine / Provider: OrbStack / Docker Desktop on Apple Silicon (linux/arm64/v8)

Steps to Reproduce

1. On an ARM64 Mac, pull or cache an amd64 alpine image:
   bash复制

   docker pull --platform linux/amd64 alpine

2. Run vcluster create:
   bash复制

   vcluster create demo

Actual Behavior

The command fails fatally with an unhelpful error message:

19:44:41 info Ensuring environment for vCluster demo...
19:44:42 fatal failed to configure network: failed to configure load balancer: failed to check if docker network is reachable: failed to inspect test network container: 
error: no such object: WARNING: The requested image's platform (linux/amd64) does not match the detected host platform (linux/arm64/v8) and no specific platform was requested
23e586d93b4ca2d51a8d21e27ad14807b1fefb4445adc275adcb03093a76b216
: exit status 1

Expected Behavior

1. vcluster should separate stdout (which contains the container ID) from stderr (which contains Docker warnings) when executing helper container commands.
2. vcluster should validate that captured container IDs match standard 12/64-character hex strings (^[a-f0-9]{12,64}$) before attempting docker inspect.
3. Optionally, vcluster should pass --platform matching the host architecture when spawning helper network test containers.

Suggested Fix

In the Docker network reachability checker package:

1. Use dedicated stdout and stderr buffers for exec.Command("docker", ...) rather than CombinedOutput().
2. Extract the container ID exclusively from stdout.
3. Provide a clear, actionable error message if container creation fails, guiding users on architecture mismatch issues.