docs: add docker compose file to help constrain zoekt
Created by: ggilmore
See https://sourcegraph.slack.com/archives/C0W2E592M/p1554755077006000
This PR adds a docker compose file to our docs that can limit the amount of CPU / RAM that Zoekt can use.
I think the issues in https://github.com/sourcegraph/sourcegraph/issues/3294 are exacerbated by Zoekt's indexing process starving out the other processes in the sourcegraph/server image (e.g. Zoekt is starving out frontend/searcher).
This alone won't fix the issue that the customer is experiencing, but adding resource isolation should help their system have more predictable performance.