added dockerfile for testnet and updated readme with examples

[lukechampine]

Thanks for the PR! However, there isn't really a need for a separate testnet Dockerfile, since you can pass the --network flag to the binary. If you keep the existing Dockerfile and the documentation for how to run it on testnet, I'll gladly merge this.

[bustedware]

@lukechampine by the time the container is running, it's either already been passed --network or it hasn't based on whats defined in the Dockerfile. Thoughts? hostd and retnerd seem to use this -tags='netgo timetzdata' but it doesn't behave the same in walletd (doesn't seem to work?) hence the extra Dockerfile. now that we have anagami this may be a moot PR. thanks