Expose the merged devcontainer configuration to the container

It would be nice if the final merged devcontainer configuration JSON were provided somewhere in the container filesystem before executing lifecycle hooks. For instance, the `postCreateCommand` for the feature of an editor tool running inside the container could refer to the `customizations` key of the merged devcontainer configuration to determine custom settings; or the `postStartCommand` for the feature of a network tool could automatically set up a tunnel to a particular port based on the `forwardPorts` key.