Add rrsync sidecar docker image

contrib/rrsync/05-hostkeys.conf

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+# Host keys are generated at container start (see entrypoint.sh) into a
+# dedicated directory rather than baked into the image or the default
+# /etc/ssh location.
+HostKey /etc/ssh/keys/ssh_host_rsa_key
+HostKey /etc/ssh/keys/ssh_host_ecdsa_key
+HostKey /etc/ssh/keys/ssh_host_ed25519_key

contrib/rrsync/10-rsync.conf

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+# Restrict all SSH sessions to rsync-over-ssh; no interactive shell access.
+ForceCommand /usr/bin/rrsync /data
+PermitTTY no
+X11Forwarding no
+AllowTcpForwarding no
+AllowAgentForwarding no

contrib/rrsync/Dockerfile

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+FROM almalinux:9
+
+LABEL maintainer="OSG Software <help@osg-htc.org>"
+
+RUN --mount=type=cache,id=dnf-9,target=/var/cache/dnf,sharing=locked \
+    dnf install -y openssh-server rsync-rrsync
+
+RUN mkdir /data
+
+RUN mkdir -p -m0755 /run/sshd && \
+    mkdir -p -m0700 /root/.ssh && \
+    mkdir -p -m0700 /etc/ssh/keys
+
+COPY --chown=root:root --chmod=0644 10-rsync.conf /etc/ssh/sshd_config.d/10-rsync.conf
+COPY --chown=root:root --chmod=0644 05-hostkeys.conf /etc/ssh/sshd_config.d/05-hostkeys.conf
+COPY --chown=root:root --chmod=0755 entrypoint.sh /entrypoint.sh
+
+CMD ["/entrypoint.sh"]
+
+EXPOSE 22/tcp
+VOLUME /data

contrib/rrsync/README.md

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+rsync-over-ssh sidecar
+======================
+
+Runs an sshd server intended to be deployed as a sidecar container in a
+Kubernetes Pod, allowing external clients to write to a shared volume via
+rsync over ssh.
+
+To permit login, volume-mount an `authorized_keys` file into
+`/root/.ssh/authorized_keys`.
+Mount the target data volume into `/data`.
+
+sshd is configured (see `10-rsync.conf`) to reject interactive shell
+access: every session is forced through [`rrsync`](https://download.samba.org/pub/rsync/rrsync.1).
+which restricts the client to rsync operations rooted at `/data` and
+rejects anything else.
+
+Example usage:
+
+```
+docker run --detach --name rrsync \
+    -v ~/authorized_keys:/root/.ssh/authorized_keys \
+    -v ~/mydata:/data \
+    -p2222:22 rrsync
+rsync -e "ssh -p 2222" myfile.txt root@localhost:/
+```

contrib/rrsync/entrypoint.sh

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+#!/bin/sh
+set -e
+
+KEY_DIR=/etc/ssh/keys
+
+if [ ! -f "$KEY_DIR/ssh_host_rsa_key" ]; then
+    for type in rsa ecdsa ed25519; do
+        ssh-keygen -q -t "$type" -f "$KEY_DIR/ssh_host_${type}_key" -N ''
+    done
+fi
+
+exec /usr/sbin/sshd -D -e