fix uwsgi installation issue on CI

[joeyorlando]

What went wrong?

What happened:
See here for example error message:
https://github.com/grafana/oncall/actions/runs/10738216009/job/29781341710#step:4:252

What did you expect to happen:

How do we reproduce it?

It only happens occasionally (but enough for it to be enough)

Grafana OnCall Version

latest

Product Area

CI/CD

Grafana OnCall Platform?

None

User's Browser?

No response

Anything else to add?

No response

[joeyorlando]

To whomever picks this up, @mderynck had an interesting idea of updating our engine Dockerfile and basically having this use a prebuilt image that we store/update on GAR (we'd probably rebuild/push this image to GAR on commits to dev?).

This image would then be used as the base image for our engine's Dockerfile, and would effectively already come with our dependencies baked-in.

This should solve our uwsgi issue as we wouldn't need to redownload Python dependencies everytime we build the image.

[joeyorlando]

I think this should be done after https://github.com/grafana/irm/issues/98 is completed