troubleshooting why docker-compose does not successfully launch locally

Normally I make changes to the repo https://github.com/allofphysicsgraph/proofofconcept/tree/gh-pages/v7_pickle_web_interface and then run git pull on my DigitalOcean VPS Droplet. It has been a long time since I tried running the web server locally on my laptop. 

Here's the process I went through to get the web server running locally.

First I had populated the "certs" directory

/Users/username/version_controlled/allofphysicsgraph/proofofconcept/v7_pickle_web_interface/certs

on my laptop from the remote VPS. 

Then I had to create 

/Users/username/version_controlled/allofphysicsgraph/proofofconcept/v7_pickle_web_interface/.env

with the Google variables for login authentication. 

In the directory on my laptop

/Users/username/version_controlled/allofphysicsgraph/proofofconcept/v7_pickle_web_interface

running the command docker-compose up --build failed. Specifically, the nginx and flask containers would start, but nginx would fail because the flask container wasn't responding. I added

restart: on-failure

to the nginx section in docker-compose.yaml

That didn't solve the flask issue, but it allowed the containers to persist while I inspected the logs. Even though I couldn't enter the flask container, I could review the logs produced by gunicorn by running the command

tail -f flask/logs/gunicorn_error.log

which showed the causal issue

  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/home/appuser/app/wsgi.py", line 15, in <module>
    from controller import app
  File "/home/appuser/app/controller.py", line 63, in <module>
    from secure import SecureHeaders  # type: ignore
ImportError: cannot import name 'SecureHeaders'
[2021-08-08 19:56:37 +0000] [11] [INFO] Worker exiting (pid: 11)
[2021-08-08 19:56:37 +0000] [1] [WARNING] Worker with pid 11 was terminated due to signal 15
[2021-08-08 19:56:37 +0000] [1] [INFO] Shutting down: Master
[2021-08-08 19:56:37 +0000] [1] [INFO] Reason: Worker failed to boot.

Root case: In my requirements.txt I hadn't pinned the version of the Python library secure. According to this issue there was a recent update . I ended up pinning secure==0.2.1 in requirements.txt

Now I am able to run docker-compose up and get a web page at https://localhost/

docker-compose version

At home on my Mac I use Docker Compose to build the combined nginx + gunicorn containers.
In the file proofofconcept/v7_pickle_web_interface/docker-compose.yaml
I had
version: "3.7"
which worked for me.
(The compatibility matrix is here: https://docs.docker.com/compose/compose-file/)

I ran docker-compose on DigitalOcean's 18.04 Ubuntu and got the message

$ docker-compose up --build --remove-orphans
ERROR: Version in "./docker-compose.yaml" is unsupported. 

The versions on DigitalOcean's Ubuntu 18.04 are

$ docker version
Client: Docker Engine - Community
 Version:           19.03.8

$ docker-compose version
docker-compose version 1.21.2, build a133471
docker-py version: 3.3.0
CPython version: 3.6.5
OpenSSL version: OpenSSL 1.0.1t  3 May 2016

while at home I have

$ docker version
Client: Docker Engine - Community
 Version:           19.03.8

$ docker-compose version
docker-compose version 1.25.4, build 8d51620a
docker-py version: 4.1.0
CPython version: 3.7.5
OpenSSL version: OpenSSL 1.1.1d  10 Sep 2019

The fix was to change the docker-compose.yaml line to
version: "3.7"