build: add range of subordinate user and group ids for build user

Some features in unprivileged user namespaces rely on the availibility
of sub uids and sub gids. Make them available for the build user in the
build environment to allow e.g. test suites that rely on this to work
correctly.

build

@@ -1728,6 +1728,11 @@ for RECIPEPATH in "${RECIPEFILES[@]}" ; do
 	    echo 'abuild:*:::::::' >>$BUILD_ROOT/etc/shadow # This is needed on Mandriva 2009
 	    echo 'abuild:*::' >>$BUILD_ROOT/etc/gshadow # This is needed on Ubuntu
 	    echo "abuild:x:${ABUILD_GID}:" >>$BUILD_ROOT/etc/group
+	    # support user namespace uids and gids, useful for test
+	    # environments that rely on unprivileged namespaces
+	    for IDFILE in subuid subgid; do
+		    echo "abuild:100000:65536" >>$BUILD_ROOT/etc/$IDFILE
+	    done
 	    mkdir -p $BUILD_ROOT/home/abuild
 	    chown "$ABUILD_UID:$ABUILD_GID" $BUILD_ROOT/home/abuild
 	else