“docker: invalid reference format” while trying to install external plugin

Installation/Setup
1

My goal is simple: Load a hello-world external plugin for Kong. I’ve decided to do this via mounted volume, but am running into significant problems that stop me from being able to run my simple hello-world external plugin.

I’ve isolated my problem down to the following commands, from scratch:

docker network create kong-net

docker run -d --name kong-database \
    --network=kong-net \
    -p 5432:5432 \
    -e "POSTGRES_USER=kong" \
    -e "POSTGRES_DB=kong" \
    -e "POSTGRES_PASSWORD=kong" \
    postgres:9.6

docker run --rm \
    --network=kong-net \
    -e "KONG_DATABASE=postgres" \
    -e "KONG_PG_HOST=kong-database" \
    -e "KONG_PG_PASSWORD=kong" \
    -e "KONG_CASSANDRA_CONTACT_POINTS=kong-database" \
    kong:latest kong migrations bootstrap

docker run -it --name kong \
    --network=kong-net \
    -e "KONG_DATABASE=postgres" \
    -e "KONG_PG_HOST=kong-database" \
    -e "KONG_PG_PASSWORD=kong" \
    -e "KONG_CASSANDRA_CONTACT_POINTS=kong-database" \
    -e "KONG_LUA_PACKAGE_PATH=/plugins/?.lua" \
    -e "KONG_CUSTOM_PLUGINS=helloworld" \ 
    -e "KONG_PROXY_ACCESS_LOG=/dev/stdout" \
    -e "KONG_ADMIN_ACCESS_LOG=/dev/stdout" \
    -e "KONG_PROXY_ERROR_LOG=/dev/stderr" \
    -e "KONG_ADMIN_ERROR_LOG=/dev/stderr" \
    -e "KONG_ADMIN_LISTEN=0.0.0.0:8001, 0.0.0.0:8444 ssl" \
    -v "kong/plugins/helloworld:/plugins" \
    -p 8000:8000 \
    -p 8443:8443 \
    -p 8001:8001 \
    -p 8444:8444 \
    kong:latest

In my local directory, I have the plugin source code sitting in ./kong/plugins/helloworld/

According to the Kong docs, I’m adjusting my Kong config file by adding “KONG_” prior to the item I want adjusted. This means I have KONG_CUMSTOM_PLUGINS=helloworld and KONG_LUA_PACKAGE_PATH=/plugins/?.lua

Docker and Kong doesn’t like this, and when running the above commands I am greeted with the following:

docker: invalid reference format.
See 'docker run --help'.

Eliminating things one-by-one, the problem surrounds both the -e "KONG_CUSTOM_PLUGINS=helloworld" and -e "KONG_LUA_PACKAGE_PATH=/plugins/?.lua" \ bits.

What’s happening here?

Thank you!