docker-compose.yml for testnet

I just created this `docker-compose.yml` file for a member of the community to connect to the testnet. It might be useful for others.

```yml
version: "3.9"
services:
hathor-core:
image: hathornetwork/hathor-core
command: run_node
ports:
- "8080:8080"
- "8081:8081"
volumes:
- /local/path/db/:/data
environment:
- HATHOR_TESTNET=true
- HATHOR_STATUS=8080
- HATHOR_STRATUM=8081
- HATHOR_X_FAST_INIT_BETA=true
- HATHOR_WALLET_INDEX=true
- HATHOR_CACHE=true
- HATHOR_CACHE_SIZE=100000
- HATHOR_DATA=/data
- HATHOR_ROCKSDB_STORAGE=true
#- HATHOR_PEER=/data/peer.json
```