Bump the minimum golang version to 1.15 in the go.mod file. The go
version is also updated in the Dockerfile and travis.yml file. This is
required so that the correct `errors.Is` method is used.
* Introduced docker integration
* Added Dockerfile to build loopd and loop with no dependencies.
* Added instructions to build and run docker image.
* Fixed documentation on mapping of ~/.lnd
* Would have caused problems because ~/.loop wasn't mapped
* Now map entire home directory
* Updated DOCKER.md example to just map ~/.lnd and ~/.loop.
* Removed mapping of home directory - one of the benefits of docker is being explicit about the resources the container uses so I felt bad mapping the entire home directory when only two known folders were used.
