repo: add LICENSE, THIRD_PARTY.md, .dockerignore, hadolint lint, IDEAS backlog

Repo/CI hygiene batch (none base-affecting; image contents unchanged):

- LICENSE: actual MIT file (repo previously declared MIT only in prose).
- THIRD_PARTY.md: notes bundled software + licenses (pi/pi-fork/pi-obsmem/
  pi-studio MIT, gosu Apache-2.0, Debian packages under their own terms).
- .dockerignore: trims build context to what the Dockerfiles COPY (rootfs/ +
  entrypoint*.sh); keeps .git/docs/scripts/compose out. Verified it excludes
  none of the required COPY sources.
- lint.yml: new hadolint job (pinned v2.14.0) lints both Dockerfiles;
  .hadolint.yaml grandfathers deliberate choices (DL3008/DL3016/DL4006/DL3003/
  SC2086, mirroring the shellcheck excludes), fails on anything new at warning+.
  Verified hadolint exit 0 and the repo shell-guard passes with the new job.
- IDEAS.md: parks deferred follow-ups (SHA-pin actions, trivy, buildx SBOM/
  provenance, Makefile, renovate).
- README/DOCKER_HUB License sections now link LICENSE + THIRD_PARTY.md.

No tag.

DOCKER_HUB.md

@@ -156,4 +156,5 @@ Optional volumes for MemPalace (commented out by default — uncomment in `docke
 
 ## License
 
-MIT (the image; pi and the bundled tools each carry their own licenses).
+MIT (the image; pi and the bundled tools each carry their own licenses). See
+`LICENSE` and `THIRD_PARTY.md` in the [source repo](https://gitea.jordbo.se/joakimp/pi-devbox).