container/ps: add HealthCheck formatter field

[Mohammed-Thaha]

- What I did

Added a new docker ps --format placeholder: .HealthCheck, so users can print container health state directly as a dedicated field.

Fixes #6203.

- How I did it

• Added HealthCheck() to the container formatter context.
• Returned health directly from c.c.Health.Status when available.
• Added fallback parsing from .Status text for:
  • (healthy) -> healthy
  • (unhealthy) -> unhealthy
  • (health: starting) -> starting
• Updated docs in docs/reference/commandline/container_ls.md to include .HealthCheck in the format placeholders table.
• Added/updated unit tests in cli/command/formatter/container_test.go.

- How to verify it

1. Start dev shell:

   make -f docker.Makefile shell

2. Build the CLI binary:

   make binary

3. Run a container that stays in starting during the start period:

   docker rm -f hc-starting 2>/dev/null || true
   
   ./build/docker-linux-amd64 run -d --name hc-starting \
     --health-cmd='exit 1' \
     --health-start-period=2m \
     --health-interval=5s \
     --health-retries=1 \
     alpine sh -c 'sleep 300'

4. Verify formatter output:

   ./build/docker-linux-amd64 ps -a --filter name=hc-starting --format "table {{.Names}}\t{{.Status}}\t{{.HealthCheck}}"

Expected result:

• STATUS contains (health: starting)
• HEALTHCHECK shows starting

- Human readable description for the release notes

`docker ps --format` now supports a `.HealthCheck` placeholder to print container health state (`starting`, `healthy`, `unhealthy`) as a dedicated field.

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