Clarify release executables' target, provide statically linked ones about docker2aci

[lucab]

As we can't cover here all possible combinations/usecases, binaries provided via GH are just shortcuts for one of the (arbitrarily chosen) most common case. As we are distributing the source here, binaries can be properly built by maintainers for any specific case (and that is the usual recommend distribution/installation mean wherever a package manager is available, like in alpine).

That said, the dynamic executable in here doesn't seem to be on purpose and could be easily turned into a static one by prepending a CGO_ENABLED=0 to the build script. If you are interested in that, would you mind submitting a short PR just doing that? That way, next release could be shipping a static executable.