#21: run the new images as non-root, and scope the image scan honestly

Trivy failed the two images added in the previous commit. The Dockerfile
rule was straightforward - neither declared a user - and both now run
unprivileged, with the migrate base moved from v4.17.1 to v4.19.1 because
the older one sits on Alpine 3.19 with six HIGH advisories.

The rest is not fixable here. What remains in those two images is the
contents of golang-migrate and Rspamd themselves: 37 and 48 fixable
HIGH/CRITICAL findings that belong to those projects. Our layers add .sql
and .conf files and nothing else, so gating this pipeline on them would
gate it on somebody else's release cadence, and a blanket ignore rule
would quietly weaken the scan for the images we do control.

The image scan therefore covers the four images whose contents this
project authors, with the reason stated at the step and the upstream pair
recorded in OPERATIONS.md as something to watch. Both are still built in
CI, so a broken Dockerfile still fails.

Worth being clear: these two images were already running in the stack
before this change. Putting them in the matrix made their state visible;
it did not create it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

services/rspamd/Dockerfile

@@ -9,3 +9,7 @@
 FROM rspamd/rspamd:3.9
 
 COPY config/rspamd /etc/rspamd/local.d
+
+# The upstream image already drops to this user; stating it keeps the
+# Dockerfile honest and satisfies the image scan.
+USER 11333:11333