List of automation improvments

[landscape82]

Patterns found in aws-vpc-terraform-plan-template and jack3-5 that spectogram's CI doesn't have yet.

• Add Markdown lint CI (markdownlint-cli2, path-filtered on **/*.md) for README.md and charts/spectogram/README.md
• Add a Docker image publish workflow on vX.Y.Z release tags (Docker Hub or GHCR) using docker/build-push-action + docker/metadata-action, so the Helm chart's image.repository/image.tag values have a real image to point at
• Add Buildx layer caching (cache-from/cache-to: type=gha) to the Docker Build CI job instead of a plain docker build
• Add concurrency groups (cancel-in-progress: true) to ci.yml, helm.yml, and release.yml so superseded runs on the same ref don't queue up
• Add timeout-minutes to CI/Helm/Release jobs to bound runaway runs
• Scope the Docker Build job in ci.yml with paths: filters (Dockerfile, docker-entrypoint.sh, cmd/**, web/**), matching the pattern already used in helm.yml