Add build env steps & standardise on git transport

[Daviey]

This change adds 3 main things:

• Previously there was a mixture of git+ssh and https transports on the submodules, and this makes it harder for non-owners to use, and requires ssh keys with access. This therefore standardises on https, which anon users can use.
• Add a Dockerfile with a build env, with the required toolchain and cross compiler for mips.
• Updated README with instructions on creating a build env (manually and using Docker).

NOTE: Whilst I have added an automated build to docker hub - I'm more than happy for my one to be dropped and replaced with a Defang managed one.

Thanks

[jmtatsch]

Thank you for your contribution.