Windows pathing error when using Dockerfile method for Docker storage

Description

When using the Dockerfile method to create storage and then register a flow with prefect cloud, there is an error COPY step that copies the flow into the Docker image.

C:\dev\scratch>python register.py
[2020-07-31 20:35:20] INFO - prefect.Docker | Building the flow's Docker storage...
Step 1/10 : FROM python:3.8.3
 ---> 659f826fabf4
Step 2/10 : RUN pip install prefect
 ---> Using cache
 ---> 79ef06e51b9f
Step 3/10 : RUN pip install pip --upgrade
 ---> Using cache
 ---> a0b4e4dec623
Step 4/10 : RUN pip show prefect || pip install git+https://github.com/PrefectHQ/prefect.git@0.12.6#egg=prefect[kubernetes]
 ---> Using cache
 ---> 00faab6426bd
Step 5/10 : RUN pip install boto3 wheel
 ---> Using cache
 ---> 7dd663167bd5
Step 6/10 : RUN mkdir -p /opt/prefect/
 ---> Using cache
 ---> ffca70689d93
Step 7/10 : COPY C:\dev\scratch\tmpk7_nwvnd\prefect-windows-pathing-repro.flow /opt/prefect/flows/prefect-windows-pathing-repro.prefect
COPY failed: stat /var/lib/docker/tmp/docker-builder087651641/C:devscratchtmpk7_nwvndprefect-windows-pathing-repro.flow: no such file or directory
Traceback (most recent call last):
  File "register.py", line 16, in <module>
    flow.register(
  File "C:\dev\scratch\venv\lib\site-packages\prefect\core\flow.py", line 1575, in register
    registered_flow = client.register(
  File "C:\dev\scratch\venv\lib\site-packages\prefect\client\client.py", line 677, in register
    serialized_flow = flow.serialize(build=build)  # type: Any
  File "C:\dev\scratch\venv\lib\site-packages\prefect\core\flow.py", line 1429, in serialize
    storage = self.storage.build()  # type: Optional[Storage]
  File "C:\dev\scratch\venv\lib\site-packages\prefect\environments\storage\docker.py", line 327, in build
    self._build_image(push=push)
  File "C:\dev\scratch\venv\lib\site-packages\prefect\environments\storage\docker.py", line 391, in _build_image
    raise ValueError(
ValueError: Your docker image failed to build!  Your flow might have failed one of its deployment health checks - please ensure that all necessary files and dependencies have been included.

This appears to be due to the fact that a Windows path is assumed to be a Posix path, so the backslashes. See above in “Step 7/10.”

Expected Behavior

The registration process should build the image without error, and finish the docker push and prefect registration processes.

Reproduction

https://github.com/aroder/prefect-windows-pathing-repro

Environment

Reproduced on 2 different Windows machines. Here are specs for one:

Optionally run prefect diagnostics from the command line and paste the information here. -->

>prefect diagnostics
{
  "config_overrides": {
    "cloud": {
      "use_local_secrets": true
    },
    "context": {
      "secrets": false
    }
  },
  "env_vars": [],
  "system_information": {
    "platform": "Windows-10-10.0.19041-SP0",
    "prefect_version": "0.12.6",
    "python_version": "3.8.3"
  }
}

[sbreidbach-edr]

Worked like magic @joshmeek .

[joshmeek]

Yeah @sbreidbach-edr that appears to be related. Do you think you could also install off of master and give it another shot like @aroder did?