[nabil1440]

Change

• FindComposeFile stops at the home directory or at the filesystem root. Before this change, a command outside $HOME did not stop and used 100% CPU.
• FindComposeFile returns the errors from os.UserHomeDir and os.Getwd. It returns ErrComposeNotFound when the site has no compose file.
• --domain must be a valid hostname: labels of letters, digits and inner hyphens, 1–63 characters each, 253 characters in total. Thus the value cannot contain / or ...
• A second check makes sure that the site directory is directly in the home directory.

Behavior change

• A domain with Unicode characters must use its punycode form (xn--…).

Tests

• Unit tests:
  • A search outside $HOME returns in less than 2 s.
  • A search from a nested directory finds the site.
  • The search stops at $HOME.
  • --domain finds the site.
  • A table of valid and invalid domains.
• End-to-end tests:
  • fly start outside $HOME stops with the no-site error.
  • --domain ../, .., example.com/../.. and a/b show "invalid domain". No Docker call occurs.
• The end-to-end tests fail on the code before this PR. The test harness stops the process after 10 s.

Closes #3