Provide a single self-contained docker image, remove alpine

[antonsviridov-src]

• Remove Alpine-based docker base as this forces us to use unofficial node builds: https://github.com/sourcegraph/sourcegraph/pull/2164/files, it might carry no risk, but also it isn't obvious to me what we gain from reduced image size
• Make docker build self-contained - it will bootstrap the version of scip-typescript it's built on, instead of relying on NPM publishing
• Start publishing docker images with :latest-snapshot on main branch builds, for easier testing and to be able to point customers to it
• Ensure we use a manfiest SHA in docker base, so that docker automatically pulls a amd64/arm64 image

Test plan

• existing CI

[kritzcreek]

Does this need a corresponding change in sg/sg to change the image we use for autoindexing?

[antonsviridov-src]

Yeah, I will update the SHA in sg/sg to the sha of the new container (single one).
We only refer to indexing containers by sha, not name (in default settings anyways)

[kritzcreek]

LGTM