Update base image, pip, add pip cache.

[ways]

Description

Solves #158

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Pull request overview

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[Copilot]

Upgrading pip without pinning can make image builds non-reproducible (pip behavior can change across releases). Consider pinning pip to a known-good version (or using a constraints file) to stabilize builds over time.

[ways]

I think pip should always be at the latest version. For other packages I agree.

[ways]

I also suggest building with something newer than python 3.9, but I don't know the background for this.