Optimize Dockerfile for Smart Contract compilation

[achimcc]

Check the currently used Dockerfile:

https://github.com/achimcc/ink-backend/blob/main/compiler/Dockerfile

Used by https://github.com/achimcc/ink-backend/blob/main/src/sandbox.rs to compile ink! Smart Contracts to WASM

Try to opmtimize the build performance. Currently the build process slows down drastically if the genereated Dockerimage is > 1 day old.

(!) Docker commands should be changed to be executed as non-root user for securioty concerns

Is Caching possible in the build process? Keep in mind that concurrent builds should be possible. Check if https://github.com/paritytech/cachepot is useful.

Tasks:

• Change to non root user: https://dzone.com/articles/docker-without-root-privileges
• Evaluate usage of cachepot: https://github.com/paritytech/cachepot
• Evaluate other performance optimizations