Include ATL_PRODUCT_HOME environmental variable in the Docker image documentation

Based on our confluence.cfg.xml.j2 jinja template, ATL_PRODUCT_HOME environmental variable can be used for overriding the default Confluence local-home path.

• https://bitbucket.org/atlassian-docker/docker-atlassian-confluence-server/src/master/config/confluence-init.properties.j2

  confluence.home = {{ atl_product_home | default(confluence_home) | default('') }}
  

This parameter is not mentioned in our Docker image documentation:

• https://hub.docker.com/r/atlassian/confluence-server/

Please include this parameter in the documentation.