In order to solve the "double notification" issue (see: https://github.com/docker/cli/commit/ef5e5fa03f0603f48678bfd7039c5b8121d98df1) without running the plugin process under a new pgid (see: https://github.com/moby/moby/issues/47073) we instead check if we're attached to a TTY, and if so skip signalling the plugin process since it will already be signalled. Signed-off-by: Laura Brehm <laurabrehm@hey.com>