podman-build: Mount /mnt/gentoo as a volume

Using a bind-mount instead of an overlay should marginally increase
performance.

podman-build.sh

@@ -12,6 +12,7 @@ cid=$(podman run \
     -v packages:/var/cache/binpkgs \
     -v distfiles:/var/cache/distfiles \
     -v packages:/usr/${target}/var/cache/binpkgs \
+    -v /mnt/gentoo \
     -v ${PWD}:/tmp/build:rw,z \
     -w /tmp/build \
     -d \