If you build a Dockerfile in your devcontainer.json that includes the following code:

ARG VARIANT="3.10"
FROM mcr.microsoft.com/devcontainers/python:0-${VARIANT}

or something similar where a string is used with quotes then you get the following error:
error: compile devcontainer.json: parse image from dockerfile: parse image ref "mcr.microsoft.com/devcontainers/python:0-${VARIANT}\r": could not parse reference: mcr.microsoft.com/devcontainers/python:0-"3.10"