Adding OnlyOffice compose

This PR adds a compose.onlyoffice.yaml file and the corresponding
configuration and description in README.md.

compose.onlyoffice.yaml

@@ -0,0 +1,48 @@
+version: "3.8"
+
+services:
+  init-onlyoffice-dirs:
+    image: busybox
+    user: root
+    command:
+      - sh
+      - -eu
+      - -c
+      - |
+        mkdir -p /cryptpad/www/common/onlyoffice/dist /cryptpad/onlyoffice-conf
+        chown -R 4001:4001 \
+          /cryptpad/www/common/onlyoffice/dist \
+          /cryptpad/onlyoffice-conf
+        exec tail -f /dev/null
+    volumes:
+      - cryptpad_oo_dist:/cryptpad/www/common/onlyoffice/dist
+      - cryptpad_oo_conf:/cryptpad/onlyoffice-conf/
+
+  app:
+    # onlyoffice-entrypoint.sh auto-chains through /sso-entrypoint.sh if
+    # compose.sso.yml is also loaded, so order of COMPOSE_FILE doesn't matter.
+    entrypoint:
+      - /onlyoffice-entrypoint.sh
+      - /cryptpad/docker-entrypoint.sh
+    environment:
+      - "CPAD_INSTALL_ONLYOFFICE=yes"
+      - ONLYOFFICE_OLDEST
+    volumes:
+      - cryptpad_oo_dist:/cryptpad/www/common/onlyoffice/dist
+      - cryptpad_oo_conf:/cryptpad/onlyoffice-conf/
+    configs:
+      - source: onlyoffice_entrypoint
+        target: /onlyoffice-entrypoint.sh
+        mode: 0755
+    deploy:
+      labels:
+        - "backupbot.backup.volumes.cryptpad_oo_dist=false"
+
+volumes:
+  cryptpad_oo_dist:
+  cryptpad_oo_conf:
+
+configs:
+  onlyoffice_entrypoint:
+    name: ${STACK_NAME}_onlyoffice_entrypoint_${ONLYOFFICE_ENTRYPOINT_VERSION}
+    file: onlyoffice-entrypoint.sh