Speed up local OSCAR image builds and add explicit no-cache rebuild support

[gmolto]

Summary

This PR speeds up the local OSCAR image build and deploy workflow without changing deployment semantics.

It improves Docker layer reuse for both the Go backend and the dashboard build, reduces the Docker build context to only the files needed for the runtime image, and adds explicit support for forcing a clean rebuild when needed.

Changes

• enable Docker BuildKit by default from the Makefile
• cache Go module downloads and Go build artifacts in the Docker build
• remove forced full recompilation flags from the Go build step
• avoid duplicate npm install work in the dashboard image build
• reorder dashboard build layers to maximize cache hits
• shrink the Docker build context with .dockerignore
• add NO_CACHE=1 support to map to docker build --no-cache

Why

Local make deploy iterations were slower than necessary because the image build repeatedly redownloaded dependencies, rebuilt layers that could be cached, and sent a much larger build context than required.

Validation

• docker build -t localhost:5001/oscar:feat-speed-build-test .
• repeated docker build reuses cached layers successfully
• make build IMAGE_TAG=feat-speed-build-make-test BUILD_CONTEXT=.
• make -n build BUILD_CONTEXT=. IMAGE_TAG=dry-run-test
• make -n build BUILD_CONTEXT=. IMAGE_TAG=dry-run-test NO_CACHE=1

Impact

The default local workflow stays the same:

• make deploy KUBE_CONTEXT=kind-oscar-test-mty

But rebuilds are significantly faster after the first successful build, and clean rebuilds remain available via:

• make deploy KUBE_CONTEXT=kind-oscar-test-mty NO_CACHE=1