test(installer): add OpenSSH distro harness

internal/installer/harness/testdata/fixtures/ubuntu/Dockerfile

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+# Ubuntu OpenSSH fixture for the source-install harness.
+#
+# No reg.rsxx.ru mirror exists for Ubuntu yet, so the default is the
+# Docker Hub ubuntu:24.04 image. Override with RSMON_TEST_IMAGE_UBUNTU.
+FROM ubuntu:24.04
+
+RUN apt-get update \
+    && DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends openssh-server \
+    && rm -rf /var/lib/apt/lists/* \
+    && mkdir -p /run/sshd /root/.ssh \
+    && chmod 700 /root/.ssh \
+    && sed -i 's/^#\?PermitRootLogin.*/PermitRootLogin prohibit-password/' /etc/ssh/sshd_config \
+    && sed -i 's/^#\?PasswordAuthentication.*/PasswordAuthentication no/' /etc/ssh/sshd_config \
+    && sed -i 's/^#\?PubkeyAuthentication.*/PubkeyAuthentication yes/' /etc/ssh/sshd_config
+
+COPY keys/rsmon_test_ed25519.pub /root/.ssh/authorized_keys
+RUN chmod 600 /root/.ssh/authorized_keys
+
+EXPOSE 22
+CMD ["/usr/sbin/sshd", "-D", "-e"]