Update docker container build script and README

Created by: fengalin

• Use https for redox apt repository
• Import the repository's key
• Update toolchain packages' names
• Install wget which is needed by coobook/recipes/xz
• Update README file to reflect the discussion about when the submodules are needed
• Update README file to suggest using make fetch all in place of make update all since Cargo.lock files come with the sources now. See this comment

Note: current source tree generation requires runing the docker container with bash -c RUSTFLAGS=\"-A unused_mut\" make fetch all in place of make fetch all