How do I list all containers in a user-defined docker network?

You can list all networks with:

docker network ls

And inspect one network to see its hosts (containers)

docker network inspect <network-id>

Inspecting that a container, you can see which network it is connected to:

docker inspect <container-id>

More info in the docs: https://docs.docker.com/engine/userguide/networking/work-with-networks/#create-networks

Another way using pure bash and no python - easier to remember for me

docker network inspect YOUR_NETWORK_ID | grep Name | tail -n +2 | cut -d':' -f2 | tr -d ',"'

Sample output

container1
container2

docker network inspect \
  -f '{{ range $key, $value := .Containers }}{{ printf "%s\n" $key}}{{ end }}' \
  <network-id>

will give a newline delimited list of the container hashes belonging to the network-id network.

You can adjust the printf format to get other syntax (comma-separated, tab-separated, container id+name, etc). For example:

 $ docker network inspect \
    -f '{{ range $key, $value := .Containers }}{{printf "%s: %s\n" $key .Name}}{{ end }}' \
    web-ingress
0b7f82ad7535ac7c1a454aaf0a5df6b87547d3cb9d751d0e0e1b4952a849b11b: mon_grafana.1.v5i0ea6nv12k0h3eo8egglg6n
1cce723642af2ce9382c7d46cca868d4674b4645d07458eeed9e928d29a4fb1f: mon_prometheus.1.lsfdcig6uhqfbbv7n07irpj3j
1f4840710f77fa1e02bd3e95581139b9f3c13fe4c857ce6ac44bfbdae4916920: mon_alertmanager.1.kwvw7kqsfpi9qzpmqdrd85yc7
2efea443fee41fd5dbca714145ca6ff95d91be9c60a469be597aadfaca90914d: mon_unsee.1.3lg8qgnvshibklnypzt5rw95s
6fdb893488f6e56766501e763d4c60196ae12a22ee9bd204d84fe324331714e8: mon_dashboard.1.r6b6nhatwmk88y5ncgagnaxh4
lb-web-ingress: web-ingress-endpoint