feat: add initial scheduled task implementation

This functionality enables users to manage a global crontab for the Dokku user that contains all scheduled tasks across all apps. Alternative schedulers can implement cron tasks as desired.

tests/apps/python/task.py

@@ -0,0 +1,5 @@
+def main(args):
+    print(args)
+
+if __name__ == '__main__':
+    main(sys.argv)

tests/unit/cron.bats

@@ -0,0 +1,95 @@
+#!/usr/bin/env bats
+
+load test_helper
+
+setup() {
+  global_setup
+  create_app
+}
+
+teardown() {
+  destroy_app
+  global_teardown
+}
+
+@test "(cron) cron:help" {
+  run /bin/bash -c "dokku cron"
+  echo "output: $output"
+  echo "status: $status"
+  assert_output_contains "Manage scheduled cron tasks"
+  help_output="$output"
+
+  run /bin/bash -c "dokku cron:help"
+  echo "output: $output"
+  echo "status: $status"
+  assert_output_contains "Manage scheduled cron tasks"
+  assert_output "$help_output"
+}
+
+@test "(cron) invalid" {
+  run deploy_app python dokku@dokku.me:$TEST_APP template_cron_file_invalid
+  echo "output: $output"
+  echo "status: $status"
+  assert_failure
+
+  run deploy_app python dokku@dokku.me:$TEST_APP template_cron_file_invalid_schedule
+  echo "output: $output"
+  echo "status: $status"
+  assert_failure
+
+  run deploy_app python dokku@dokku.me:$TEST_APP template_cron_file_invalid_schedule_seconds
+  echo "output: $output"
+  echo "status: $status"
+  assert_failure
+}
+
+
+template_cron_file_invalid() {
+  local APP="$1"
+  local APP_REPO_DIR="$2"
+  [[ -z "$APP" ]] && local APP="$TEST_APP"
+  echo "injecting invalid cron app.json -> $APP_REPO_DIR/app.json"
+  cat <<EOF >"$APP_REPO_DIR/app.json"
+{
+  "cron": [
+    {
+      "key": "value"
+    }
+  ]
+}
+EOF
+}
+
+template_cron_file_invalid_schedule() {
+  local APP="$1"
+  local APP_REPO_DIR="$2"
+  [[ -z "$APP" ]] && local APP="$TEST_APP"
+  echo "injecting invalid cron app.json -> $APP_REPO_DIR/app.json"
+  cat <<EOF >"$APP_REPO_DIR/app.json"
+{
+  "cron": [
+    {
+      "command": "python task.py",
+      "schedule": "@nonstandard"
+    }
+  ]
+}
+EOF
+}
+
+template_cron_file_invalid_schedule_seconds() {
+  local APP="$1"
+  local APP_REPO_DIR="$2"
+  [[ -z "$APP" ]] && local APP="$TEST_APP"
+  echo "injecting invalid cron app.json -> $APP_REPO_DIR/app.json"
+  cat <<EOF >"$APP_REPO_DIR/app.json"
+{
+  "cron": [
+    {
+      "command": "python task.py",
+      "schedule": "0 5 * * * *"
+    }
+  ]
+}
+EOF
+}