bind SFTP on an ephemeral port of the Docker host

Trying to bind 22 can be problematic; for one thing, it's tripping up our
CI/CD. Note that the containers address the original port number when speaking
to each other directly.

envs/sftp-mysql-example/docker-compose.yml

@@ -9,7 +9,7 @@ services:
   ftp-server:
     image: iowoi/sftp-server:latest
     ports:
-        - 22:22
+        - "22"  # Bind on an ephemeral port, user `docker ps` to find out where it landed.
     env_file:
       - sftp.env
 
@@ -30,6 +30,6 @@ services:
       - ${ENVIRONMENT_FOLDER}/whirl.setup.d:${WHIRL_SETUP_FOLDER}/env.d/
       - ${DAG_FOLDER}/whirl.setup.d:${WHIRL_SETUP_FOLDER}/dag.d/
       - ${MOCK_DATA_FOLDER}:/mock-data
-    depends_on:
+    links:
       - ftp-server
       - mysql

examples/sftp-mysql-example/README.md

@@ -29,4 +29,6 @@ The environment to be used is set in the `.whirl.env` in the DAG directory. In t
 
 The DAG contains 2 tasks that use:
 - The SFTPOperator to copy the file from the FTPserver to the local directory
-- The Python operator using the MySQL Hook to load the csv data into the database.
+- The Python operator using the MySQL Hook to load the csv data into the database.
+
+The SFTP server is exposed to the host on an ephemeral port. Run `docker ps` to find out the local port number.