test(installer): add OpenSSH distro harness

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

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+# 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"]

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

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+# 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"]

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"]

internal/installer/harness/testdata/keys/rsmon_test_ed25519

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+-----BEGIN OPENSSH PRIVATE KEY-----
+b3BlbnNzaC1rZXktdjEAAAAABG5vbmUAAAAEbm9uZQAAAAAAAAABAAAAMwAAAAtzc2gtZW
+QyNTUxOQAAACCz7h3HFwJrC+DIrE3W+9yI+hRAcCVesyEQmCicmPrqAwAAAKgIO12YCDtd
+mAAAAAtzc2gtZWQyNTUxOQAAACCz7h3HFwJrC+DIrE3W+9yI+hRAcCVesyEQmCicmPrqAw
+AAAEDqbSMcuhF56miNJOZKUOuA/9I6yVga06nirb7pns41lLPuHccXAmsL4MisTdb73Ij6
+FEBwJV6zIRCYKJyY+uoDAAAAIHJzbW9uLXdvcmtlciBzb3VyY2UtaW5zdGFsbCB0ZXN0AQ
+IDBAU=
+-----END OPENSSH PRIVATE KEY-----

internal/installer/harness/testdata/keys/rsmon_test_ed25519.pub

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+ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAILPuHccXAmsL4MisTdb73Ij6FEBwJV6zIRCYKJyY+uoD rsmon-worker source-install test