Remove shell directive

podman complains about shell directive:
SHELL is not supported for OCI image format, [/bin/bash -o pipefail -c]
will be ignored. Must use `docker` format
1 job for master in 8 minutes and 45 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #386946
podman ppc64le-linux
build-ppc64le-linux-opensuse151

00:08:45