Improve docs for docker installation

[letzya]

https://tyk.io/docs/get-started/with-tyk-on-premise/installation/docker/

In case users need to open ports for Redis and mongodb, it doesn't declare it clearly on the docker hub https://hub.docker.com/r/tykio/tyk-gateway/, https://hub.docker.com/r/tykio/tyk-dashboard. Since it's docs for installing separate containers we should detail the ports of Redis and mongo on our docs or in docker hub.

[letzya]

@marksou

[marksou]

@letzya I don't have access to edit the Docker Container page. I've added the info in the relevant GitHub Repos. @lonelycode Can you change the Docker Containers to reflect the changes I've made in the Github repos for the Gateway, Dashboard and Pump.

[stale]

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