Feature Request: Support for unregistered packages

[ivborissov]

It would be great to extend SimpleContainerGenerator.jl support to unregistered packages. Currently one can set pkgs argument to SimpleContainerGenerator.create_dockerfile as pkgs = [(name = "MyUnregisteredPackage.jl", url = "https://...")] and name and url will be correctly put into Pkg.PackageSpec in the generated install_packages.jl script. However build process fails on the following line of install_packages.jl script:

for (uuid, info) in Pkg.dependencies()
  Pkg.add(info.name)
end

ERROR: LoadError: expected package MyUnregisteredPackage to be registered

[urlicht]

+1 on this. It'd be great to be able to include a pkg from private remote repo or a local directory