Add build-linux target

[alpe]

We do not provide a build-linux target for cross compilation.
The issue we had in the past with wasmd was that the result had dynamic linked libraries. This is something that we should avoid.
Terra had bad experience with this and uses a docker container to build the artifact.

We could do the same with our Dockerfile and copy the tgrade binary from an instance with docker cp <container_name>:/usr/bin/tgrade . after building . Other solutions can be discussed