Name builder instance to avoid multiple ones on self-hosted runners

[whoan]

Close #151

If you are running self-hosted runners, you may need to clean some stale builder instances created by v8.0.0 of the action. To do so, you can run something like this (review and use with caution):

docker buildx ls \
    | grep -E "^[^ ].+ docker-container" \
    | while read -r instance_name rest; do docker buildx rm "$instance_name"; done

FYI @milandufek ^

v8.0.2 was released with the fix