build: Fix check for ubuntu:21.10

The check for ubuntu:21.10 that changes the repo path doesn't work with
podman. We need to cope with the fact that the $from includes the
"docker.io/" prefix.

Also to see the value of $from we need to reintroduce it after the
actual FROM directive, see:
  https://docs.docker.com/engine/reference/builder/#understand-how-arg-and-from-interact#understand-how-arg-and-from-interact

Author: mpe

build/ubuntu/Dockerfile

@@ -1,5 +1,6 @@
 ARG from
 FROM ${from}
+ARG from
 
 COPY ubuntu/packages.sh /tmp/packages.sh
 
@@ -8,7 +9,7 @@ ENV apt_mirror=${apt_mirror}
 RUN [ -n "$apt_mirror" ] && sed -i -e "s|ports.ubuntu.com|$apt_mirror|" /etc/apt/sources.list || true
 
 # Deal with Ubuntu 21.10 repos moving to old-releases (ppc64le only)
-RUN [ "$from" = "ubuntu:21.10" ] && sed -i -e "s|ports.ubuntu.com/ubuntu-ports|old-releases.ubuntu.com/ubuntu|" /etc/apt/sources.list || true
+RUN if echo "$from" | grep -q ".*ubuntu:21.10"; then sed -i -e "s|ports.ubuntu.com/ubuntu-ports|old-releases.ubuntu.com/ubuntu|" /etc/apt/sources.list; fi
 
 RUN apt-get -q -y update && \
     DEBIAN_FRONTEND=noninteractive \