Provide good env var defaults in the binary instead of Dockerfile

[emidoots]

We should move all env vars from the Dockerfile into the binary itself so that consumers using just the binary (e.g. Sourcegraph) do not have to separately maintain their own list of env vars to configure Rocket.

Should be as simple as setting them if not already set.

[emidoots]

Now that we have http-server-stabilizer in use, too, it would make more sense to use a shell script instead. This would allow us to more easily get http-server-stabilizer working in sourcegraph/server deployments, as well.