Improve services health check

What does this MR do?

Improves services health check for Docker executor:

• makes output more clear and more verbose (especially it shows health check container logs, so exit code 1 is not such enigmatic anymore)
• fixes the way how service container is linked to health check container (which in some cases, especially when service's image contains dots in the name, generate false alarms)

Why was this MR needed?

Please look into gitlab-selenium-server#7 (closed) and #3184 (closed) for examples of such problem.

Are there points in the code the reviewer needs to double check?

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?

Closes gitlab-selenium-server#7 (closed), #3184 (closed)