Heredocs in Dockerfiles dont work and show up as blank inside container when building images

[brandon-wick]

building images with the docker SDK for python using Dockerfile's with heredocs show up as blank inside container.
Python code to reproduce

import docker
import os
import json

def build_docker_image():
    client = docker.from_env()
    # print(json.dumps(client.version(), indent=4))
    # print(json.dumps(client.info(), indent=4))
    image, _ = client.images.build(path='/Users/wick/DockerTest', platform="linux/amd64",, tag='rocky8-test:latest')
    print(f"Image {image.tags[0]} built successfully!")
    for log in _:
        print(log)

if __name__ == '__main__':
    build_docker_image()

Dockerfile used:

FROM rockylinux:8

RUN cat <<EOF > /tmp/test_file
test content
test content
test content
EOF

Results:
/tmp/test_file will be blank.

Additional info:
client.version() output:

{
    "Platform": {
        "Name": "Docker Desktop 4.28.0 (139021)"
    },
    "Components": [
        {
            "Name": "Engine",
            "Version": "25.0.3",
            "Details": {
                "ApiVersion": "1.44",
                "Arch": "amd64",
                "BuildTime": "2024-02-06T21:14:25.000000000+00:00",
                "Experimental": "false",
                "GitCommit": "f417435",
                "GoVersion": "go1.21.6",
                "KernelVersion": "6.6.16-linuxkit",
                "MinAPIVersion": "1.24",
                "Os": "linux"
            }
        },
        {
            "Name": "containerd",
            "Version": "1.6.28",
            "Details": {
                "GitCommit": "ae07eda36dd25f8a1b98dfbf587313b99c0190bb"
            }
        },
        {
            "Name": "runc",
            "Version": "1.1.12",
            "Details": {
                "GitCommit": "v1.1.12-0-g51d5e94"
            }
        },
        {
            "Name": "docker-init",
            "Version": "0.19.0",
            "Details": {
                "GitCommit": "de40ad0"
            }
        }
    ],
    "Version": "25.0.3",
    "ApiVersion": "1.44",
    "MinAPIVersion": "1.24",
    "GitCommit": "f417435",
    "GoVersion": "go1.21.6",
    "Os": "linux",
    "Arch": "amd64",
    "KernelVersion": "6.6.16-linuxkit",
    "BuildTime": "2024-02-06T21:14:25.000000000+00:00"
}