build-setup: support podman

We have some Redhat servers I'm interested in using for OpenBMC builds.
With just the minor change in this commit, I was able to get openbmc to
successfully build on a RH8 system which points "docker" to the podman
executable.

The failure without this commit is the following:
```
Error: runc: runc create failed: unable to start container process: exec: "/home/geissonator/89239843/build.sh": stat /home/geissonator/89239843/build.sh: permission denied: OCI permission denied
```

Some online searching brought me to this --userns parameter. Setting
this env variable fixed the issues on the RH8 system.

Tested:
- Confirmed openbmc will build on RH8 system with podman
- Confirmed openbmc still builds on Ubuntu system

Change-Id: I3d1127c25dd08a1c8748943f8f3a1add11fe12ee
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
diff --git a/build-setup.sh b/build-setup.sh
index 8f21910..53af227 100755
--- a/build-setup.sh
+++ b/build-setup.sh
@@ -412,6 +412,10 @@
     mount_workspace_dir=""
 fi
 
+# If we are building on a podman based machine, need to have this set in
+# the env to allow the home mount to work (no impact on non-podman systems)
+export PODMAN_USERNS="keep-id"
+
 # Run the Docker container, execute the build.sh script
 # shellcheck disable=SC2086 # mount commands word-split purposefully
 docker run \