Move Dockerfile.frontend, .backend, .exporter, .mcp and .storybook
under docker/images/ from ubuntu:26.04 / nginx-unprivileged / a
manual Node tarball install to Docker Hardened Images (Debian 13,
or Alpine for storybook). storybook and mcp get a true non-dev
runtime; frontend, backend and exporter keep the -dev tag as their
final image, since each needs a shell and/or package manager at
container runtime (nginx templating, fontforge/python3, and a
headless-browser stack, respectively).
The notify steps referenced mattermost/action-mattermost-notify@master, a
mutable branch that runs in CI with access to the MATTERMOST_WEBHOOK_URL
secret. Pinning to the immutable commit of the latest release (v2.1.0,
ae31bb6) keeps the exact reviewed code from executing, per GitHub third-party
action hardening guidance, while staying easy to bump.
