Installation instructions for Docker

I'm following the instructions provided at the CKAN documentation for 2.9.3 for installing extensions with docker-compose, since it uses ckanext-spatial as an example. I'm not able to complete the process though.

The problems begin right after the documentation states that "Some extensions require database upgrades, often through paster scripts. E.g., ckanext-spatial".

Then comes two lines of code that do not work as they should:

`docker exec -it db psql -U ckan -f 20_postgis_permissions.sql
`
This first one throws a "`no such file 20_postgis_permissions.sql`" error.

I managed to fix this by asking for direction at the gitter channel. So it got changed to this:

`docker exec -it db psql -U ckan -f /docker-entrypoint-initdb.d/20_postgis_permissions.sql
`
The second one though, still not working:

`docker exec -it ckan /usr/local/bin/ckan-paster --plugin=ckanext-spatial spatial initdb -c /etc/ckan/production.ini
`
It gives the error:

`OCI runtime exec failed: exec failed: container_linux.go:380: starting container process caused: exec: "/usr/local/bin/ckan-paster": stat /usr/local/bin/ckan-paster: no such file or directory: unknown
`