How can i access the Open Policy Agent Command Line via Docker Desktop in Windows 10 - docker-compose

Question

I am attempting to learn the various features of something called Open Policy Agent because I think it may be a useful tool in a microservices based application.
Here is a link to the 'Running with Docker' section of the documentation for this application: https://www.openpolicyagent.org/docs/latest/deployments/#running-with-docker
Currently, I am running Docker using the Docker Desktop in a Windows 10 environment and I already have a docker-compose file set up for my main application which includes various docker images. My thoughts were that I could simply add the latest openpolicyagent image as well as the openpolicyagent demo-restful api so that I could begin learning about the service. To do this, I added the following lines to my docker-compose.yml:
opa:
image: openpolicyagent/opa:0.34.2
ports:
- 8181:8181
command:
- "run"
- "--server"
- "--log-level=debug"
- "api_authz.rego"
volumes:
- C:\Sites\prosaurus\policy\api_authz.rego:/api_authz.rego
api_server:
image: openpolicyagent/demo-restful-api:latest
ports:
- 5000:5000
environment:
- OPA_ADDR=http://opa:8181
- POLICY_PATH=/v1/data/httpapi/authz
This appears to have worked in that I can go to localhost:8181 and i see the Query and Input Data (JSON) boxes as I presume is supposed to happen, however I would like to test some of the command line functions as are mentioned here:
https://www.openpolicyagent.org/docs/latest/#2-try-opa-eval
However I can not seem to access the command line of the docker container which is running the OPA agent. The way I have attempted this is via the Docker Desktop application GUI in Windows. In this application I can see all of the docker instances which are running and each one has an option to run the CLI (you click the button and the cli opens). They all work except for the OPA one. When I click on that one a cmd window opens for a split second, displays something too fast for me to read it and then closes.
What have I done wrong?

Answer 1

OPA can be run in a few different ways, and opa eval is distinctly different from running OPA as a server, i.e. opa run --server.
When you run OPA as a server - which is how you'd normally run OPA in production - you query OPA for policy decisions through OPA's REST API.
opa eval on the other hand is more like a Swiss army knife of OPA, allowing you to quickly evaluate a rule or expression given some provided policy and data.
You can think of them as two entirely different tools.