[Remote Development] Devcontainer support without internet access

I work for a financial organization, where its work environment has very limited access to internet, for risk control reason. 

When setting up remote development with SSH, there is optional to specify custom backend package URL (an ideaU file uploaded to intranet). So no internet is required.

However, when setting up remote development with Devcontainer, there doesn't seem to have an option to specify custom backend package URL, or manual upload… it always tries to download from internet, and always fails to do so due to the block.

Is there a workaround to this?