Cf. https://github.com/julmon/pg_activity
It is a very small image (~129MB uncompressed) available on docker hub based on Alpine Linux and using the last available release from the official PyPI repo of pg_activity.
docker run --rm -it tophfr/pg_activity --help
docker run --rm -it -e PGHOST=mypghost -e PGUSER=mypguser tophfr/pg_activity