bug: The TTY detection identifies /dev/null as a terminal #12

Blocker: no. This can wait. Do this with the Docker capability.

Defect

• isInteractive (internal/docker/compose.go:15-18) examines only stdout. It uses os.ModeCharDevice.
• /dev/null is a character device. Thus, isInteractive returns true for /dev/null. A test gave this result.
• Cron jobs frequently send output to /dev/null. For these jobs, the CLI does not add -T to docker compose exec.
• The function does not examine stdin.

Cost

• With Docker Compose v5.1.1, a cron-style call with stdin and stdout on /dev/null operated correctly. Thus, the effect is small on current Compose versions.
• The effect on older Compose versions is not known.

Benefit of the fix

• The CLI identifies a terminal correctly in all conditions.
• The behavior does not depend on the Compose version.

Fix

Use go-isatty on stdin and stdout. go-isatty is already in the dependency tree.