docker automated build

[pierrehpezier]

build.sh will automatically build the release for linux x86 using docker && docker-compose.

[cretz]

Neat, thanks. I had considered a Dockerfile at the root (I'd use multistage and/or put the shell script in the container instead of calling commands via compose, and I might use alpine instead of ubuntu). For now, since the steps are straightforward and cross-platform, I think I'll leave the Dockerfile out for now. But I'll leave this PR here for anyone that wants it.

[cretz]

Nice update to use alpine and do the build inside the container there! Still not really wanting to merge, but this is helpful. Also note that, while the docker concerns itself with stripping and upx'ing the Tor binary at the end, this repo is more about the static libraries (i.e. the .a files produced). I will leave this PR open for anyone else that wants the docker file.