Update devcontainer to be a prebuilt container

[RossBugginsNHS]

What is the problem this feature will solve?

Improve devcontainer start time

What is the feature that you are proposing to solve the problem?

create a separate dockerfile which is a prebuilt version of the current devcontainer, publish the container as an artifact, update devcontainer to just use that container image.

What alternatives have you considered?

No response

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