Update Dockerfile with extensions support

[mgoin]

Created the nm-vllm-openai package: https://github.com/neuralmagic/nm-vllm/pkgs/container/nm-vllm-openai

Adds a new manual docker build+publish workflow that will build a docker image using the Dockerfile at the top-level and push it to ghcr.io/neuralmagic/nm-vllm-openai. It will upload two tags, one with the current date and one with latest for ease of use.

I also manually built a docker image for the v0.1.0 branch by checking out that branch, applying the magic-wand extensions, and running:

docker build --tag ghcr.io/neuralmagic/nm-vllm-openai:v0.1.0 --target vllm-openai .
docker push ghcr.io/neuralmagic/nm-vllm-openai:v0.1.0