Be able to pass environment variables in containers start()

[gabivlj]

We are just defining in the container capnp schema the environmentVariables property, and the env property in the start options.
The user will be able to pass environment variables like this:

this.ctx.container.start({ 
  env: {
    A: "C",
    C: "A",
  },
});

If the container image is already configured to have A=C, it will override it and set A to B. New environment variables are just appended. It's up to the container runtime to implement this behaviour correctly.

[kentonv]

Looks good but you need to run the formatter.

[gabivlj]

rebased