ci: add Ubuntu instructions for s390x-self-hosted-builder

There is a new Ubuntu-based builder, and setup steps for it are
slightly different from what we had on the old RHEL-based one.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>

travis-ci/rootfs/s390x-self-hosted-builder/README.md

@@ -10,7 +10,8 @@ the master branch.
 ### Install prerequisites.
 
 ```
-$ sudo dnf install docker
+$ sudo dnf install docker        # RHEL
+$ sudo apt install -y docker.io  # Ubuntu
 ```
 
 ### Add services.
@@ -53,7 +54,8 @@ get the latest OS security fixes, use the following commands:
 $ sudo docker build \
       --pull \
       -f actions-runner-libbpf.Dockerfile \
-      -t iiilinuxibmcom/actions-runner-libbpf
+      -t iiilinuxibmcom/actions-runner-libbpf \
+      .
 $ sudo systemctl restart actions-runner-libbpf
 ```