btw, I tried with alpine instead of ubuntu:18.04, got :

Unable to find image 'alpine:latest' locally
latest: Pulling from library/alpine
df20fa9351a1: Pulling fs layer
df20fa9351a1: Verifying Checksum
df20fa9351a1: Download complete
df20fa9351a1: Pull complete
Digest: sha256:185518070891758909c9f839cf4ca393ee977ac378609f700f60a771a2dfe321
Status: Downloaded newer image for alpine:latest
docker: Error response from daemon: OCI runtime create failed: container_linux.go:349: starting container process caused "exec: "bash": executable file not found in $PATH": unknown.
time="2020-06-09T13:24:55Z" level=error msg="error waiting for container: context canceled"
DONE: Running task 'ae788658786043ef8d3c758815a43eb8', exit status 127

Any idea?