Add docker image for ceph-rbd backups

ceph-rbd-backup/Dockerfile

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+FROM quay.io/ceph/ceph:v20.2
+
+COPY script.sh /script.sh
+
+ENTRYPOINT ["/bin/bash", "/script.sh"]

ceph-rbd-backup/README.md

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+# Ceph-RBD-backup
+
+## Environment Variables
+| Name | Default | Description |
+|---|---|---|
+| CEPH_USERNAME | - | TODO |
+| RBD_POOL | - | TODO |
+| RBD_IMAGE_ID | - | TODO |
+| SNAPSHOT_ID | - | TODO |
+| OUTPUT_FILE | - | TODO |

ceph-rbd-backup/script.sh

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+#!/usr/bin/env bash
+
+set -euo pipefail
+
+CEPH_USERNAME="${CEPH_USERNAME:-}"
+RBD_POOL="${RBD_POOL:-}"
+RBD_IMAGE_ID="${RBD_IMAGE_ID:-}"
+SNAPSHOT_ID="${SNAPSHOT_ID:-}"
+OUTPUT_FILE="${OUTPUT_FILE:-}"
+
+# Create the snapshot
+rbd --name ${CEPH_USERNAME} snap create ${RBD_POOL}/${RBD_IMAGE_ID}@${SNAPSHOT_ID};
+
+# Export the snapshot to a file
+rbd --name ${CEPH_USERNAME} export --export-format 2 ${RBD_POOL}/${RBD_IMAGE_ID}@${SNAPSHOT_ID} - | zstd -T1 -3 ${OUTPUT_FILE};
+touch ${OUTPUT_FILE}.done;
+
+ls -al ${OUTPUT_FILE};
+
+# Remove the snapshot again
+rbd --name ${CEPH_USERNAME} snap rm ${RBD_POOL}/${RBD_IMAGE_ID}@${SNAPSHOT_ID};