[AMQ-9296] Add support of credentials to the docker image

[essobedo]

Fixes https://issues.apache.org/jira/browse/AMQ-9296

Motivation

Since Docker images for Apache ActiveMQ are published, it would be great to support env vars for USER and PASSWORD.

Modifications:

• Add an entry point script to generate the configuration files if the environment variable ACTIVEMQ_USER is set
• Describe the supported environment variables

[jbonofre]

I'm not sure adding an authenication plugin is what's expected.
IMHO, the user (in Jira) is asking for authentication on the webconsole, so updating jetty-realm.properties. The same can apply to credentials.properties and users.properties. I can explain to you the use of those different files.
So I think, we could have different env variables, depending the credentials we want to set.

[essobedo]

I understand the differences with the authentication on the web console, I simply believed that it is what the OP was expecting. I guess the OP needs to clarify a bit more because it is really unclear/misleading

[jbonofre]

Agree, the ticket should provide some details. However, I think it would be acceptable to have different env variables, depending what kind of credentials we want to provide.

[jbonofre]

Superseded by #1114