Fix executable not in $PATH in Dockerfile

[gayanW]

This fixes:

docker run --rm -e SMART_TESTS_TOKEN=$SMART_TESTS_TOKEN cloudbees/smart-tests-cli:v2.5.0 verify
docker: Error response from daemon: failed to create task for container: failed to create shim task: OCI runtime create failed: runc create failed: unable to start container process: error during container init: exec: "smart-tests": executable file not found in $PATH

Key changes:

• Change to multi-stage build for smaller image size
• Install the package system-wide using a wheel file instead of installing it into a venv (similar to v1)