better way to decide what deps go in the sysimage

[kolia]

This should result in fewer sysimage builds during docker builds, which is by far the longest build step generally.

Instead of basing the subset of deps included in the sysimage off of pinned packages, this:

• includes all deps that track a registry
• saves a ~Manifest.toml.julia_pod~ julia_pod/sysimage.packages listing dependencies to possibly include in the sysimage, and use it in a way that allows for packages to be added without invalidating the cached sysimage layer.

The rationale for this change is that:

• managing long lists of pinned deps is cumbersome
• pinned direct deps do not protect transitive deps from changing versions, even if they are pinned by hand in the Manifest
• one most often wants to add all deps to the sysimage anyway, minus deps one is hacking on, which are generally deps that track a path or a repo.
• the docker build cache was being invalidate too often

Edit: This also defines and makes use of a startup.jl file, which gets copied to .julia/config/startup.jl in the pod, and saves the sysimage.jl and startup.jl files to a new local julia_pod/ folder alongside julia_pod/sysimage.packages, for hygene.

closes #37

[kolia]

what happens if Manifest.toml.julia_pod contains a different version of a dep than Manifest.toml?

A dep that is in both but with different versions would keep the version in Manifest.toml, invalidate the docker cache and trigger a sysimage re-build.

Edited: ~Hmm, in that case, the Manifest.toml.julia_pod should be overwritten with the new Manifest.toml to reflect the new deps in the sysimage, let me fix that.~ This is no longer an issue, since I am now only listing sysimage package names, instead of saving the full Manifest.toml that contains version info.

[kolia]

Ready for re-review.