Implement DISABLE_UPDATES environment variable for PMM Server Docker container

Implement DISABLE_UPDATES environment variable for PMM Server Docker container. "-e DISABLE_UPDATES=1" or "-e DISABLE_UPDATES=true" (strconv.ParseBool() in Go code) should:

1. Hide Update option from the UI;
2. Return error if customer tries to run update by any means.

Note: we don't have ACLs in PMM yet so no other check is required.

from Peter Zaitsev email

I know in theory Docker container suppose to be immutable and docker purrists would hate that but we have docker way for them.
I think however we should have the option (variable?) which can disable update functionality while starting docker container - if someone wants to do it manually.