fix: change base docker image to allow application startup

[demeringo]

See #320

This works but there may be more optimized solutions.

[da-ekchajzer]

Yeah ! Works for me. I think we used python-slim at first because the docker image was too big. Does this new base image still keep the image small ?

[demeringo]

I am pretty sure it is not good, we can compare and get insights with dive https://github.com/wagoodman/dive

Just we have to keep in mind that we compare differents versions (different deps).

[demeringo]

@da-ekchajzer I checked and the result is terrible: 1,7 GB 💣 (compared to around 500 MB before), so yes we clearly have to optimise before merge.

[demeringo]

I made a typo in the previous dockerfile which lead to reuse the same image in a 2 stages build.

Last commit separates images in the 2 stages and brings better results: 665 MB.

It can and should still be optimized but IMHO we could use this version for the time being.