Move docker/imagemagick/Dockerfile and docker/devenv/Dockerfile from
ubuntu:26.04 to Docker Hardened Images (Debian 13 / trixie).
imagemagick gets a true non-dev runtime with its shared libraries
vendored via ldd; devenv keeps the -dev tag as its final image since
it's an interactive development container, not a production
artifact.
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.
