Introduce a self-contained, OIDC-free e2e harness that stands up NetBird
in containers, so suites no longer depend on the hand-maintained Tilt
stack or a real IdP.
- harness brings up the combined server (management + signal + relay +
STUN + embedded IdP) in a single container built from
combined/Dockerfile.multistage, and mints an admin PAT through the
unauthenticated /api/setup bootstrap (NB_SETUP_PAT_ENABLED). API access
goes through the existing shared/management/client/rest typed client.
- the image is built via the docker CLI (BuildKit) so the Dockerfile's
cache mounts are honored; testcontainers then runs the tagged image.
- everything is behind the `e2e` build tag so normal builds and unit
tests never pull in testcontainers.
Adds BuildKit cache mounts to combined/Dockerfile.multistage so source
changes recompile incrementally rather than from scratch.
Pillar 1 proven by TestCombinedBootstrap: server builds, boots, mints a
PAT, and the PAT authenticates a real management API call.
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