Compose is failing if dockerfile specified in the context's path

[jmthvt]

Description of the issue

Compose is failing if a dockerfile is specified in the context's path.

Context information (for bug reports)

docker-compose version 1.21.0, build unknown
docker-py version: 3.2.1
CPython version: 2.7.14
OpenSSL version: OpenSSL 1.0.2o  27 Mar 2018

Client:
 Version:	18.04.0-ce
 API version:	1.37
 Go version:	go1.10.1
 Git commit:	3d479c0
 Built:	unknown-buildtime
 OS/Arch:	darwin/amd64
 Experimental:	false
 Orchestrator:	swarm

Server:
 Engine:
  Version:	18.03.0-ce
  API version:	1.37 (minimum version 1.12)
  Go version:	go1.9.4
  Git commit:	0520e24
  Built:	Wed Mar 21 23:14:32 2018
  OS/Arch:	linux/amd64
  Experimental:	true

networks:
  devops-test: {}
services:
  devops-test:
    build:
      context: /Users/johndoe/repos/test/devops-test/
      dockerfile: /Users/johndoe/repos/test/devops-test/dockerfile/Dockerfile
    environment:
    image: repo.dkr.ecr.eu-west-1.amazonaws.com/devops-test
    networks:
      devops-test: null
version: '3.0'

Steps to reproduce the issue

Just run docker-compose build with a declared dockerfile in the context.

Observed result

Building devops-test
ERROR: Cannot locate specified Dockerfile /Users/johndoe/repos/test/devops-test/dockerfile/Dockerfile

Expected result

docker-compose -f tools/docker/docker-compose.yml build
Building devops-test
Step 1/21 : FROM alpine:3.7
3.7: Pulling from library/alpine

Stacktrace / full error message

(if applicable)

Additional information

macOS 10.13.3

This is following up #5823

[shin-]

Ok, I can reproduce that one. Note that in this case though, you can express the Dockerfile path relative to the context path, i.e.

version: '3'
services:
  test:
    build:
      context: /Users/johndoe/repos/test/devops-test/
      dockerfile: ./dockerfile/Dockerfile
    networks:
      - devops-test

networks:
  devops-test:

and it will work.