test(installer): add OpenSSH distro harness

internal/installer/harness/testdata/fixtures/arch/Dockerfile

@@ -0,0 +1,19 @@
+# Arch Linux OpenSSH fixture for the source-install harness.
+#
+# No reg.rsxx.ru mirror exists for Arch yet, so the default is the
+# Docker Hub archlinux:latest image. Override with RSMON_TEST_IMAGE_ARCH.
+FROM archlinux:latest
+
+RUN pacman -Sy --noconfirm --needed openssh \
+    && mkdir -p /run/sshd /root/.ssh \
+    && chmod 700 /root/.ssh \
+    && ssh-keygen -A \
+    && 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"]