Consider changing the Docker image used for the dev container to `ehrql` image

[StevenMaude]

Could we make this based on ehrql:v1

The ms version doesn't nothing that is useful to us, afaict:
https://github.com/devcontainers/images/blob/main/src/python/.devcontainer/Dockerfile

Might require some work to test and customise the entrypoint for devcontainer usage, but might be a good idea?

Might also be a good idea for research-template-docker, too perhaps, depending on how the R issue is resolve, as we have a similar version mismatch thing there.

Originally posted by @bloodearnest in #27 (comment)

[StevenMaude]

(I created the issue as it may be useful for us to link to that when updating a current research-template-docker issue which deals with the same topic, as Simon mentioned.)