The composite action didn't work as expected (the action's
`action.sh` and `build.sh` weren't accessible), so I replaced the
shell script with a tiny JavaScript bootstrap, that will properly
collect the action inputs and generate a simple local build.sh and
calls it.

Note that we couldn't use a Docker container action, because we
couldn't access the host docker ending, and would need to add both
docker cli and docker server to the docker container...