Document the differences in devcontainer specification implementation

[kylecarbs]

e.g. mounts aren't supported

[evilhamsterman]

@kylecarbs why is this even an issue, you removed documentation about this yourself in commit ca6eccf. I understand that it's a fast moving project and the support may be changing rapidly but having at least some of the information would be helpful

[kylecarbs]

The support section I removed in that commit wasn't very complete, so it felt weird to mention some things that aren't supported.

I agree this info is helpful - we'll prioritize this.

[evilhamsterman]

I think the way it was documenting what subset you do support would be a good way to deal with it.

[kylecarbs]

Reasonable, that's how we'll do it.

[matifali]

Are there plans to support mounts? cc: @bpmct An example devcontainer that uses mounts and I am unable to build with envbuilder is: https://github.com/railwayapp/nixpacks

[mafredri]

It's a bit early to tell, but there is a chance that the solution we land on for #128 may enable passing the list of volumes to Terraform, which can then be made available for the template author. However, it's a bit unclear to me if we can turn it into a robust implementation that is easily usable between different cloud providers and container runtimes.

[mafredri]

Updated list or differences:

• No support for volumes (yet)
• Feature init scripts (entrypoint) are not supported (yet)

Please add more as they are discovered.