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* Optimize container images for startup

This change adjusts how to handle runner payloads to support
container builds where we keep them extracted in the filesystem.
This makes it easier to optimize the cpu/cuda vs cpu/rocm images for
size, and should result in faster startup times for container images.

* Refactor payload logic and add buildx support for faster builds

* Move payloads around

* Review comments

* Converge to buildx based helper scripts

* Use docker buildx action for release

If a VERSION is not specified, this will generate a version string that
represents the state of the repo.  For example `0.1.21-12-gffaf52e1-dirty`
representing 12 commits away from 0.1.21 tag, on commit gffaf52e1
and the tree is dirty.

This also refines the build process for the ext_server build.

Run the server.cpp directly inside the Go runtime via cgo
while retaining the LLM Go abstractions.