Simplify devcontainer

[bpkroth]

Pull Request

Title

Combines container build steps to reduce devcontainer size substantially.

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Description

Reduces container size from ~5.7GB to ~2.7GB by combining the following steps:

• install conda (previously in a base image layer)
• create the base mlos environment

This reduces the space substantially because conda's usual attempt to do hardlinks across environments is actually able to work.

The downside is that changes to base package level requirements require reinstalling all of conda again (i.e., the single large combined container layer is less cacheable).

Should help with issues like: Microsoft-CISL/sqlite-autotuning#7

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Type of Change

• 🔄 Refactor
• Dev environment

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Testing

• time make devcontainer
• "Rebuild devcontainer" inside VSCode
• docker image ls to check the sizes

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