feat: add health-check flag to allow user defined health checks

[travis-mcginley]

Changelog

• Enabled the ability for a user to define a health check via a flag
• Updated documentation with new flags and an example

---

Ref: #93, #81

[travis-mcginley]

Example of usage:

12:57:37 whoami ±|expose-health-check ✗|→ docker inspect whoami-test --format='
Status: {{.State.Health.Status}}
Failing Streak: {{.State.Health.FailingStreak}}
{{range .State.Health.Log}}
Time: {{.Start}}
Exit Code: {{.ExitCode}}
Output: {{.Output}}
---{{end}}'

Status: healthy
Failing Streak: 0

Time: 2025-07-22 12:55:39.327049875 -0400 EDT
Exit Code: 0
Output: Health check passed


---
Time: 2025-07-22 12:56:09.359296369 -0400 EDT
Exit Code: 0
Output: Health check passed


---
Time: 2025-07-22 12:56:39.400734482 -0400 EDT
Exit Code: 0
Output: Health check passed


---
Time: 2025-07-22 12:57:09.437798349 -0400 EDT
Exit Code: 0
Output: Health check passed


---
Time: 2025-07-22 12:57:39.470122932 -0400 EDT
Exit Code: 0
Output: Health check passed

[op3]

I would suggest to squash some (or even all?) of the commits to clean up the git history a bit.

Looking forward to this being merged!