The following Pipeline code produces an error:

def image = docker.image("any_existing_image_eg_alpine")
image.inside {
    sh 'date > /tmp/test.txt'
    sh 'cat /tmp/test.txt' //works, shows file

    def fileContents = readFile '/tmp/test.txt' //works
    echo "Contents: ${fileContents}" //works, shows file

    echo "Exists: ${fileExists('/tmp/test.txt')}" //FAILS (returns false)
    archiveArtifacts '/tmp/*.txt' //FAILS the build
}

archiveArtifacts always reports that it matched no files (except in $WORKSPACE, see below).

DSL steps in docker summary (what I've tried):

• readFile(): works correctly on files in or out of $WORKSPACE
• writeFile(): works correctly on files in or out of $WORKSPACE
• fileExists(): works correctly only on files in $WORKSPACE (auto-mounted volume)
• archiveArtifacts(): archives only files in $WORKSPACE  (auto-mounted volume) 

So I guess there is a pattern ... and yet the behavior is inconsistent.  I could not find any documentation that explains what the rules are.

UPDATE: readFile() now throws NoSuchFileException (docker-pipeline 1.17 and pipeline-basic-steps 2.7) for files outside the workspace, which is probably better.