How we can install contentaCMS in pantheon?
One way might be using a custom upstream ( https://pantheon.io/docs/create-custom-upstream ). The other possible way might be creating a sandbox and then setting it to an empty one and using it that way ( https://pantheon.io/docs/guides/composer-convert#change-upstreams )
I’m not super familiar with how contentaCMS works so those are some possible ideas. Let us know if that helps out in any way though.