Add support for building in docker

[LamaAni]

As a developer I would like to build woodpecker on any environment

Added:

1. Make command to build woodpecker on docker (recreate make build environment).
2. Dockerfile for make environment

[anbraten]

Isn't this somehow redundant to the work done in #635? We at least use a similar approach for building the cli and agent already.

[6543]

No its a different one ... beside that xgo use docker but thats a tooling specific to croscimpile cgo

[6543]

also is it possible to preserver the permissions from user exec that command. at the moment build artefacts are owned by root if run within docker :/

[6543]

make version
> next-22ae7fb8
VERSION=1.0 make version
> 1.0
make in_docker version # expect same as before get ""
>