[nabil1440]

Change

• Each command uses RunE. Execute shows the error one time and sets the exit status.
• When docker compose or wp-cli fails, fly exits with the same status and shows no more text. For example, wp plugin is-installed keeps its exit status.
• Other errors go to stderr as Error: … with exit status 1. Usage errors also show Run 'fly <command> --help' for usage.
• This PR removes the 8 color.Red("…:", err) calls. They showed %!(EXTRA …).
• Success messages show only after success.
• sites start, sites stop and sites restart continue after a failed site. At the end, they show each failed site. One function replaces the two copies of this code.
• Each failure line starts with its phase, for example stopping example.com: exit status 1. Thus sites restart shows two different lines when a site fails to stop and then fails to start.
• fly exec without arguments and fly update without root give exit status 1.
• internal/testutil adds a fake docker script for tests. main_test.go builds fly and runs it end to end.

Behavior change

• Scripts that call fly now get a correct exit status. Before this change, most failures gave exit status 0.

Tests

• Unit tests: exitCode, forEachSite, and sites restart with a site that fails in the two phases.
• End-to-end tests with the fake docker:
  • A child exit status of 7 gives a fly exit status of 7.
  • Ten Docker commands exit with the child status 3 and show no success message.
  • The no-site error goes to stderr.
  • Usage errors give exit status 1.
• The same end-to-end tests fail on the code before this PR.
• Real Docker (Compose v5.1.1): fly exec -- php sh -c 'exit 7' exits with status 7.

Closes #2