docker: Support multiple Ethereum networks

This splits up the `ethereum` environment variable into multiple strings
and passes each value on to graph-node as a separate `--ethereum-rpc`
argument. Prior to this, a value like

```
ethereum: "mainnet:... ropsten:..."
```

would be passed in as a single `--ethereum-rpc` argument and would fail
to parse.

Author: Jannis

docker/start-node

@@ -5,5 +5,5 @@ set -x
 
 graph-node \
   --postgres-url "postgresql://$postgres_user:$postgres_pass@$postgres_host/$postgres_db" \
-  --ethereum-rpc "$ethereum" \
+  --ethereum-rpc $(echo $ethereum) \
   --ipfs "$ipfs"