The following command (found in the plone-backend README) fails for me:
docker run -p 8080:8080 plone/plone-backend:6.0.0a1 start
I get this error
docker: Error response from daemon: OCI runtime create failed: container_linux.go:380: starting container process caused: exec: "start": executable file not found in $PATH: unknown. ERRO error waiting for container: context canceled
If I leave out the 'start'
docker run -p 8080:8080 plone/plone-backend:6.0.0a1
It works as expected.