MT#62249 Refresh docker image

We need the latest ngcp-archive-keyring version, addressing the SHA-1
situation with our repository key.  To get that, we refreshed our base
image sipwise-trixie, but need to ensure that we refresh all the docker
images building on top of it.

While at it, update the ENV syntax to avoid the deprecation warning.

Change-Id: Id4a048370260102400e7703addfa323ed7b761e6

t/Dockerfile

@@ -5,7 +5,7 @@ FROM docker.mgm.sipwise.com/sipwise-trixie:latest
 # is updated with the current date. It will force refresh of all
 # of the base images and things like `apt-get update` won't be using
 # old cached versions when the Dockerfile is built.
-ENV REFRESHED_AT 2025-12-30
+ENV REFRESHED_AT=2026-01-30
 
 # files that get-code generates
 COPY t/sources.list.d/builddeps.list /etc/apt/sources.list.d/