Why not just have:

&& chown -R odoo /var/lib/odoo \
&& chown -R odoo /mnt/extra-addons

That is what I am using (without checking for the existence of the directory first, like you're doing in line 43) .. and it works just fine (and resolves 404 issues when I add all the enterprise modules)

Also there should be no need to check if the dir exists because according to Docker documentation "Volumes are initialized when a container is created"

When you build the image docker creates intermediary containers at which point the volume would be initialized.