Move postAttach and postCreate actions to Dockerfile

[Jongmassey]

The research template devcontainer.json contains a postCreateCommand setting and postAttachCommand setting. Currently the former calls .devcontainer/postCreate.sh (which is included in the research template repo), and the latter starts rstudio server.

If we wish to make changes to what happens on either of these events, then this currently requires making changes to the research template which:

• is harder to deploy to all users
• is harder to test that concurrent changes to the Dockerfile and devcontainer.json work correctly together

I propose that:

• postCreate.sh, requirements.in and a new postAttach.sh are added to the image as part of the Dockerfile
• the devcontainer.json postCreateCommand and postAttachCommand settings call these scripts.
• postcreate.sh and requirements.in are removed from the research template repository

Acceptance criteria

• The postAttach and postCreate scripts are moved as described above
• Documentation is updated for adding a Codespace to an existing project
• There's an upgrade path for existing Codespaces users.

Note: we should keep an eye on how long this work takes and maybe reconsider if it potentially takes us beyond 28th June.

[StevenMaude]

Repository changes

• Add more dev container files to the Docker image #57
• Use Docker image files in dev container config opensafely/research-template#137

Documentation changes

I don't think that the documentation for adding Codespaces to a project needs updating The steps to update are the same, and there are still multiple files to copy across to a project.

If we need to describe the upgrade path, the simplest way to do so would be:

1. Delete the existing files out of .devcontainer in your project.
2. Repeat the steps for "Adding Codespaces to your project".

PR to explain the upgrade path (which isn't documented):

• Explain updating Codespaces config for a project opensafely/documentation#1547

[StevenMaude]

I think merging the PRs above should be sufficient to close this issue.

[StevenMaude]

Closing as the related PRs are now merged.