enhance devcontainer.json for prebuilds, az installs

[nitya]

Is your feature request related to this sample app, or to an Azure service, such as Azure OpenAI or Azure AI Search?
Describe the scope of your request. This forum is focused on features relating to this sample app and its functionality, not the functionality of underlying Azure services.

Yes, related to this sample.

Is your feature request related to a problem? Please describe.
A clear and concise description of what the problem is. Ex. I'm always frustrated when

Using GitHub Codespaces to launch takes a long time.
We also get the documented issue on "az" needing to be updated manually sometimes.
Both these should be easy to resolve with devcontainers config.

Describe the solution you'd like
A clear and concise description of what you want to happen.

We should enable prebuilds on the devcontainer for speedup.
We should have the curl -sL https://aka.ms/InstallAzureCLIDeb | sudo bash command run post-create by default in the prebuild, so the container is always setup right.

Is this feature specific to your use case or your organization, or would it apply broadly across other uses of this app?
Describe how you and others might leverage this feature.

Will apply broadly to anyone who takes the devcontainers route to explore sample

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Alternative is to (a) wait for non-prebuilds, and (b) do installs manually
Both take up valuable time especially in time-constrained lab environments.

Additional context
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