I have a localstack docker container running, and several other containers/apps running alongside of it. I am not sure what, if any, are using it though. How do I figure out if anything has a dependency upon this container?
It is hard to tell which container “uses” another container. The connection could be direct like referring to the other container’s name as domain name somehere in a config file or referring to an external domain that points back to a host port which is forwarded to a container. Files / volumes could be shared among containers, kernel namespaces could be shared like network or process namespaces for example.
You can try checking the output of docker container inspect CONTAINERNAME and look for clues, but you would also need to check all the config files or not likely but possibly the source code of the apps in the containers.
Depending on how production that system is, you can try sopping containers and see if the other containers are still running properly and webinterfaces or APIs are working. But without a documentation or someone who remembers what is connected to what, it is not easy to find which container depends on which other containers.
Inspect the Docker network to see which containers are connected alongside LocalStack. You can also review your docker-compose.yml or container configs for any references to LocalStack endpoints.