openssh_server: build container for arm64 also, fix Dockerfile for trixie

To enable testing on arm64 Linux and possibly also on macOS, and
possibly possibly on arm64 Windows.

Also:
- switch to debian:testing to trigger a container rebuild.
- Dockerfile: install `adduser` explicitly. Required for debian:forky.
- Dockerfile: stop installing suggested/recommended packages.

Ref: https://github.com/docker/build-push-action

Closes #2606

.github/workflows/openssh_server.yml

@@ -77,9 +77,18 @@ jobs:
           tags: |
             type=raw,value=${{ steps.hash.outputs.hash }}
 
+      - name: 'install prereqs'
+        if: ${{ steps.poll.outcome == 'failure' }}
+        run: |
+          sudo find /etc/apt/sources.list.d -type f -not -name 'ubuntu.sources' -delete -print
+          sudo apt-get -o Dpkg::Use-Pty=0 update
+          sudo apt-get -o Dpkg::Use-Pty=0 install \
+            qemu-user-static
+
       - uses: docker/build-push-action@53b7df96c91f9c12dcc8a07bcb9ccacbed38856a # v7.3.0
         if: ${{ steps.poll.outcome == 'failure' }}
         with:
+          platforms: linux/amd64,linux/arm64
           context: ./tests/openssh_server
           push: true
           tags: ${{ steps.meta.outputs.tags }}

tests/openssh_server/Dockerfile

@@ -2,10 +2,10 @@
 # SPDX-License-Identifier: BSD-3-Clause
 
 # To update, get the latest digest e.g. from https://hub.docker.com/_/debian/tags
-FROM debian:trixie-20260824-slim@sha256:d7e12182ce18b85b93007c1dedf31f2d29e01ccf3182cc4017c709b6259bc132
+FROM debian:testing-20260824-slim@sha256:b21cba5194c0cddc19b41045ceebce11b5413ac40bac7f2410c066b5acb28091
 
 RUN apt-get update \
- && apt-get install -y openssh-server \
+ && apt-get install -y --no-install-suggests --no-install-recommends adduser openssh-server \
  && apt-get clean \
  && rm -rf /var/lib/apt/lists/*
 RUN [ -d /var/run/sshd ] || mkdir /var/run/sshd