Fall back to default behavior when deployed to a worker node (#593)

* Add test demonstrating failure when running on worker node

* Consider manager status when checking for swarm related features

* Update documentation

test/worker-node/.multinode

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+worker1

test/worker-node/docker-compose.yml

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+services:
+  database:
+    image: mariadb:10.7
+    deploy:
+      restart_policy:
+        condition: on-failure
+      placement:
+        constraints:
+          - node.hostname == worker1
+    environment:
+      MARIADB_ROOT_PASSWORD: test
+      MARIADB_DATABASE: backup
+    labels:
+      - docker-volume-backup.archive-pre=/bin/sh -c 'mysqldump -ptest --all-databases > /tmp/volume/dump.sql'
+      - docker-volume-backup.copy-post=/bin/sh -c 'echo "post" > /tmp/volume/post.txt'
+      - docker-volume-backup.stop-during-backup=true
+    volumes:
+      - app_data:/tmp/volume
+
+  other_database:
+    image: mariadb:10.7
+    deploy:
+      placement:
+        constraints:
+          - node.hostname == manager
+      restart_policy:
+        condition: on-failure
+    environment:
+      MARIADB_ROOT_PASSWORD: test
+      MARIADB_DATABASE: backup
+    labels:
+      - docker-volume-backup.archive-pre=touch /tmp/volume/not-relevant.txt
+      - docker-volume-backup.exec-label=not-relevant
+    volumes:
+      - app_data:/tmp/volume
+
+  backup:
+    image: offen/docker-volume-backup:${TEST_VERSION:-canary}
+    deploy:
+      restart_policy:
+        condition: on-failure
+      placement:
+        constraints:
+          - node.hostname == worker1
+    environment:
+      BACKUP_FILENAME: test.tar.gz
+      BACKUP_CRON_EXPRESSION: 0 0 5 31 2 ?
+      EXEC_FORWARD_OUTPUT: "true"
+    volumes:
+      - backup_archive:/archive
+      - app_data:/backup/data:ro
+      - /var/run/docker.sock:/var/run/docker.sock:ro
+
+volumes:
+  app_data:
+  backup_archive:

test/worker-node/run.sh

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+#!/bin/sh
+
+set -e
+
+cd $(dirname $0)
+. ../util.sh
+current_test=$(basename $(pwd))
+
+export TMP_DIR=$(mktemp -d)
+export LOCAL_DIR=$(mktemp -d)
+
+while [ -z $(docker ps -q -f name=backup) ]; do
+  info "Backup container not ready yet. Retrying."
+  sleep 1
+done
+
+sleep 20
+
+docker exec $(docker ps -q -f name=backup) backup
+
+mkdir -p /archive
+docker cp $(docker ps -q -f name=backup):/archive $LOCAL_DIR
+
+tar -xvf "$LOCAL_DIR/archive/test.tar.gz" -C $TMP_DIR
+if [ ! -f "$TMP_DIR/backup/data/dump.sql" ]; then
+  fail "Could not find file written by pre command."
+fi
+pass "Found expected file."
+
+if [ -f "$TMP_DIR/backup/data/post.txt" ]; then
+  fail "File created in post command was present in backup."
+fi
+pass "Did not find unexpected file."