extraDirectories should use container user:group or have a parameter to set the user:group instead of using root

Environment:

• Jib version:
• Build tool: gradle 7.5.1
• OS: ubuntu

Description of the issue: I'm writing this as a bug, because I couldn't find it on documentation or internet search or in the issue list, if there is already something feel free to send me the related documentation or example.

If a user is set on container section, it should be use by extraDirectories or extraDirectories should be able to accept a user:group as parameter, today extraDirectories is forcing ownership to 'root'.

I have an application that is run with a dedicated user:group (not root) and I am adding files with extraDirectories. Using "myuser:mygroup" as example.

With a configuration such as (I have anonymized enterprise data):

jib {
    from {
        image = //an custom docker image
    }
    container {
        user = "myuser:mygroup"
        ports = ["8080"]
    }
    extraDirectories {
        paths {
            path {
                from = file("build")
                into = "/opt/myuser/app"
            }
        }
    }
    pluginExtensions {
        pluginExtension {
            implementation = "com.google.cloud.tools.jib.gradle.extension.ownership.JibOwnershipExtension"
            configuration {
                rules {
                    rule {
                        glob = "/opt/myuser/**"
                        ownership = "myuser:mygroup"
                    }
                }
            }
        }
    }
}

Expected behavior: Being able to have files created on runtime as "myuser" inside /opt/myuser/app.

But I have permissions errors because user on the 'app' folder is 'root' instead of the expected 'myuser', such as:

[myuser@6213a8c424b3 app]$ pwd
/opt/myuser/app
[myuser@6213a8c424b3 app]$ touch myfile.txt
touch: cannot touch 'myfile.txt': Permission denied
[myuser@6213a8c424b3 app]$ ls -lart
-rw-r--r-- 1   myuser myuser    93 Jan  1  1970 version.txt
drwxr-xr-x 3 myuser myuser  4096 Jan  1  1970 bin
drwxr-xr-x 1 root   root    4096 Jan  1  1970 ..
drwxr-xr-x 1 root   root    4096 Jan  1  1970 .

Additional information is that /opt/myuser/app already exists in the base docker image, if I remove the extraDirectories block then the user is myuser on /opt/myuser/app (so it's forced changed ownership to root by JibOwnershipExtension).

Steps to reproduce:

1. Build a docker image with gradle jib, having a user:group different from root
2. Copy some file in a folder '/opt/app' with JibOwnershipExtension
3. Run docker image as user different from root, try to "touch myfile.txt" inside '/opt/app'

Additional Information:

As I read limitations on https://github.com/GoogleContainerTools/jib-extensions/blob/6b81680d0882cfd5c3bd38c8a1c7034d53854107/first-party/jib-ownership-extension-gradle/README.md?plain=1#L60-L64

The workaround is to have the app folder inside the build folder of gradle, and have this =>

extraDirectories {
        paths {
            path {
                from = file("build/") //this one has build/app/bin
                into = "/opt/myuser"
            }
        }
    }

But if a user is set on container section, it should be use by extraDirectories or extraDirectories should be able to accept a user:group as parameter, I don't see any security reason to force having 'root' user, since it's more a good practice to NOT run applications as root in docker.