Introduce DSL for controlling custom Docker containers

[pun-ky]

right now, GAP is controlling HTTP container during environmentUp command, but the approach could be more generic. Let buildscript users to allow control other services during making environment up and restarting during environmentClean

[pun-ky]

e.g Knot.x (custom container) may be controlled using this DSL / if required https://github.com/Cognifide/gradle-aem-multi/pull/65

[pun-ky]

DSL proposal

aem {
    environment {
        containers {
            httpd { // or 'override("httpd")'
                up {
                    // override default behavior
                }
                clean {
                    // override default behavior
                }
            }
            "knotx" { // or 'define("knotx")'
                up {
                    // exec any init commands needed
                }
                clean {
                    exec("sudo service knotx restart") // sth like that
                }
            }
        }
    }
}

up callbacks will be called during environmentUp task and clean callbacks accordingly when environmentClesn task is running

[pun-ky]

final dsl