[BUG] docker commands in README don't work

Created by: mdavis-xyz

Bug Report Checklist

Have you provided a full/minimal spec to reproduce the issue?
[n/a] Have you validated the input using an OpenAPI validator (example)?
What's the version of OpenAPI Generator used?
Have you search for related issues/PRs?
What's the actual output vs expected output?
[Optional] Bounty to sponsor the fix (example)

Description

openapi-generator version

openapi-generator-cli 5.0.0-SNAPSHOT
  commit : 068ad02
  built  : 2020-07-02T17:04:18Z
  source : https://github.com/openapitools/openapi-generator
  docs   : https://openapi-generator.tech/

OpenAPI declaration file content or url

see below

Command line used for generation

see below

Steps to reproduce

Go to the development-in-docker section of the readme.
Follow the instructions, copy-pasting in each command

expected result

The commands run sucessfully and generate code.

actual result

$ ./run-in-docker.sh generate -i modules/openapi-generator/src/test/resources/2_0/petstore.yaml     -g go -o /gen/out/go-petstore -DpackageName=petstore
++ dirname ./run-in-docker.sh
+ cd .
+ maven_cache_repo=/home/ec2-user/.m2/repository
+ mkdir -p /home/ec2-user/.m2/repository
++ id -u
++ id -g
+ docker run --rm -it -w /gen -e GEN_DIR=/gen -e MAVEN_CONFIG=/var/maven/.m2 -e 'MAVEN_OPTS=-Dhttps.protocols=TLSv1.2 -Dmaven.repo.local=/var/maven/.m2/repository -Dorg.slf4j.simpleLogger.log.org.apache.maven.cli.transfer.Slf4jMavenTransferListener=WARN -Dorg.slf4j.simpleLogger.showDateTime=true -Djava.awt.headless=true -Djacoco.skip=true' -u 501:501 -v /home/ec2-user/environment/dispatch_swagger/docker/openapi-generator:/gen -v /home/ec2-user/environment/dispatch_swagger/docker/openapi-generator/CI/run-in-docker-settings.xml:/var/maven/.m2/settings.xml -v /home/ec2-user/.m2/repository:/var/maven/.m2/repository --entrypoint /gen/docker-entrypoint.sh maven:3-jdk-8 generate -i modules/openapi-generator/src/test/resources/2_0/petstore.yaml -g go -o /gen/out/go-petstore -DpackageName=petstore
[error] Found unexpected parameters: [-DpackageName=petstore]

See 'openapi-generator-cli help' for usage.

Related issues/PRs

N/A

Suggest a fix

The documentation should clarify how to pass in options like packageName in the command line. Including on pages like this