Use environment variable for global opt-out and Docker Desktop (if
available) to determine specific experiment states.
In the future, we'll allow per-feature opt-in/opt-out via env vars
as well, but currently there is a single `COMPOSE_EXPERIMENTAL` env
var that can be used to opt-out of all experimental features
independently of Docker Desktop configuration.
This is a good place to start introducing (local) exclusivity
to Compose. Now, when `alpha watch` launches, it will check for
the existence of a PID file in the user XDG runtime directory,
and create one if the existing one is stale or does not exist.
If the PID file exists and is valid, an error is returned and
Compose exits.
A slight tweak to the experimental remote Git loader has been
made to use the XDG package for consistency.
Signed-off-by: Milas Bowman <milas.bowman@docker.com>
Kirill A. Korinsky
Milas Bowman
Guillaume Lours
Nicolas De Loof