Cannot locate specified Dockerfile in 1.20.1

[mnebuerquo]

Description of the issue

I have some scripts I use for devops, which include some shell scripts and docker-compose to build, lint, test, and run a python project. It's been working great until a recent update. Now it says "Cannot locate specified Dockerfile", but then prints out the correct full path to the Dockerfile.

I have created a repo for reproducing this issue. It contains a very stripped-down replica of my devops scripts, and a directory structure to reproduce this issue. It also contains logs of the build command in two different environments to show expected and actual.

See the example repo here: https://github.com/mnebuerquo/compose-example

Observed result

Cannot locate specified Dockerfile: /opt/compose-example/compose/dockerfile/Dockerfile

Expected result

The container builds successfully in 1.18.0.

Additional information

This is the docker-compose.yml after the environment variables have been expanded:

# /opt/compose-example/compose/docker-compose.yml
version: '3'
services:
  dev:
    build:
      context: /opt/compose-example/context
      dockerfile: /opt/compose-example/compose/dockerfile/Dockerfile

Related

I think this might be related to one of these issues:
#5869
#4857

[shin-]

Duplicate of #5869