Fix local container sidecar routing

[iGmainC]

What this fixes

Fixes #6790.

Local container startup could time out when the container egress sidecar was enabled. The failure showed up as repeated readiness-check aborts and, in some cases, a kj/timer.c++:30 timeout before the application container appeared usable from the Worker.

Root cause

In local Docker mode, workerd previously used the sidecar's host-published ingress port for several local control and data paths:

• reading the sidecar CA certificate from /ca
• updating the sidecar /egress configuration
• connecting to application container ports through DockerPort::connect()

That host-to-published-port route is not reliable with proxy-everything's transparent proxying rules. Host-originated traffic to the published sidecar ingress port can be intercepted by the sidecar's TPROXY setup and hang instead of reaching the intended sidecar control endpoint or application port.

The application container and the Sandbox server can be healthy, but workerd's local readiness path still times out because it is probing through the affected host-published sidecar route.

Solution

This changes the local Docker implementation to avoid host-originated requests to the sidecar's published ingress port:

• read the sidecar CA certificate with Docker's archive API from /ca/ca.crt
• update /egress by running a small Docker exec inside the sidecar and sending the request to 127.0.0.1:39001 from inside that container
• bridge application port connections by running nc 127.0.0.1:<port> inside the sidecar network namespace, where 127.0.0.1:<port> reaches the application container because it shares the sidecar network namespace

Docker exec attach streams are multiplexed when TTY is disabled, so the port bridge now wraps the stream and exposes stdout frames as the raw TCP byte stream.

Tests

• bazel test //src/workerd/server:container-client-test@
• bazel build //src/workerd/server:workerd
• Verified the minimal Sandbox terminal reproduction with the rebuilt workerd binary. The WebSocket terminal opens and returns command output from /workspace.

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[iGmainC]

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