fix(docker): pin numeric uid/gid so hadolint DL3066 passes

hadolint 2.15.x - pulled in by hadolint-action v3.5.0 - adds DL3066
("Non-numeric user-id may not be resolvable by host system"). The
Dockerfile's `USER muser` trips it and the hadolint job exits 1 (hadolint
fails on info-level findings by default), turning the PR Checks pipeline
red on the Renovate bump PR.

muser was created without an explicit uid/gid, so its numeric identity
depended on busybox's auto-assignment. Both ids are now pinned at
1000:1000 - the values Alpine was already assigning - and USER uses the
numeric form, which is always resolvable by the host.

Verified with the upstream hadolint v2.15.1 binary: the previous
Dockerfile reports "Dockerfile:9 DL3066 info" and exits 1, the fixed one
lints clean and exits 0.

The hadolint-action v3.3.0 -> v3.5.0 bump is included here (all four
workflows, SHA 06be81ba) so that master ends up on v3.5.0 with a clean
lint - superseding Renovate PR #23.

Dockerfile

@@ -4,8 +4,10 @@ LABEL maintainer="Cabillot Julien <dockerimages@cabillot.eu>"
 # hadolint ignore=DL3018
 RUN apk add --no-cache mysql-client && \
     rm -rf /usr/share/apk/keys && \
-    adduser -D muser
+    addgroup -g 1000 muser && \
+    adduser -D -u 1000 -G muser muser
 
-USER muser
+# Numeric uid/gid (DL3066): identity stays resolvable by the host
+USER 1000:1000
 
 ENTRYPOINT [ "/usr/bin/mysql" ]