* Docker fragment for Bitcoin Wallet Tracker
* Add docs for bwt
* Persist all BWT_* options to BTCPAY_ENV_FILE
* Add bwt to DockerFileBuildHelper, remove dynamic image name
The dynamic image didn't play nicely with the build helper, so it was
removed in favour of only supporting the Electrum variant for now.
* Update to bwt v0.2.2 with multi-arch images
