Add support for podman build --group-add

Signed-off-by: Daniel J Walsh <dwalsh@redhat.com>

docs/source/markdown/podman-build.1.md.in

@@ -287,6 +287,20 @@ expected. For example, oci-archive:/x.tar will reference /x.tar on the remote
 machine instead of on the client. When using podman remote clients it is
 best to restrict use to *containers-storage*, and *docker:// transports*.
 
+#### **--group-add**=*group* | *keep-groups*
+
+Assign additional groups to the primary user running within the container
+process.
+
+- `keep-groups` is a special value that tells Buildah to keep the supplementary
+group access.
+
+Allows container to use the user's supplementary group access. If file systems
+or devices are only accessible by the rootless user's group, this flag tells the
+OCI runtime to pass the group access into the container. Currently only
+available with the `crun` OCI runtime. Note: `keep-groups` is exclusive, other
+groups cannot be specified with this flag.
+
 #### **--help**, **-h**
 
 Print usage statement