I'm trying to set a custom PATH in a docker.image('...').inside block.

For example, I would like to be able to do something like this:

node('docker') {
    docker.image('some-build-image').inside {
        sh 'echo $PATH'
        withEnv(['PATH+MAVEN=/opt/maven-3.3.3/bin']) {
            sh 'echo $PATH'
            sh 'mvn --version'
        }
    }
}

But the PATH environment variable inside the docker image does not get updated - the two echo statements produce exactly the same output, and the Maven command fails with the following error: "mvn: command not found"

I see that as a result of #JENKINS-43590, the PATH env var is no longer passed from the host to the docker container (which seems sensible, as the environments can be different), but I feel it should still possible to manipulate the PATH variable inside the docker container somehow, e.g by using withEnv. Even a workaround like running the shell step sh 'export PATH=$PATH:/opt/maven-3.3.3/bin' does not have the required outcome.