For anybody like me that needs a SSL connection to the database in this form:
"postgresql://joplin:passw@private-db-postgresql.db.ondigitalocean.com:25061/joplin_pool?sslmode=require"
- name: APP_BASE_URL
value: "https;//joplin.example.com"
- name: APP_PORT
value: "22300"
- name: DB_CLIENT
value: "pg"
- name: POSTGRES_CONNECTION_STRING
value: "postgresql://joplin:passw@private-db-postgresql.db.ondigitalocean.com:25061/joplin_pool?sslmode=require"
- name: MAILER_ENABLED
value: "0"
- name: NODE_TLS_REJECT_UNAUTHORIZED
value: "0"
to get the ssl connection work you need to set NODE_TLS_REJECT_UNAUTHORIZED: 0