test(installer): add OpenSSH distro harness

internal/installer/harness/testdata/fixtures/alpine/Dockerfile

@@ -0,0 +1,21 @@
+# Alpine OpenSSH fixture for the source-install harness.
+#
+# Base image is the reg.rsxx.ru mirror (alpine:3). openrc is installed
+# explicitly so init-system detection has a stable marker; openssh is the
+# minimal sshd. Password auth is disabled; the fixture authenticates with
+# the shared test key.
+FROM reg.rsxx.ru/library/alpine:3
+
+RUN apk add --no-cache openssh openrc \
+    && 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"]