add docker support

[llitfkitfk]

[mtojek]

Hi @llitfkitfk

Thank you very much for your input! Shall I ask you to fix the Travis build as well?

[mtojek]

Another comment, any reason why you've enabled "go mod"? I wouldn't say no, but not as a part of this PR.

[llitfkitfk]

go mod for docker build download dependencies and build greenwall command

[llitfkitfk]

if some dependencies have breaking changes, docker build could be failed I think

[mtojek]

Generally l'm fine with your changes, but please fix the build on Travis, then I will merge it. Thanks!

[llitfkitfk]

I think go mod need added the first time

[mtojek]

Ok @llitfkitfk, so kindly please prepare a PR (with go mod only) that passes the Travis build.

BTW Later on, you can update README file with section about Docker.