title: "file not found" on travis-ci when running pytest from tox using docker
body: <p>I'm trying to run my tests throught travis-ci, but i receive
  "file not found tests" error. When i run local with same command everything is ok, but in travis i receive this error. I think its because tests folder which in root of my project located somewhere in other directory - in directory where travis copy github repo.</p>
  
  <p>I try this settings in <code>tox.ini</code> but none of whem help:</p>
  
  <pre><code>commands = py.test $TRAVIS_BUILD_DIR/tests {posargs}
  
  passenv = TRAVIS_BUILD_DIR
  commands = py.test $TRAVIS_BUILD_DIR/tests {posargs}
  
  commands = py.test {env:TRAVIS_BUILD_DIR:.}/tests {posargs}
  </code></pre>
  
  <p>My setup:</p>
  
  <p>My <code>.travis.yml</code></p>
  
  <pre><code>sudo: required
  
  language: python
  
  services:
    - docker
  
  script:
    - docker-compose run tox
  </code></pre>
  
  <p>My <code>docker compose file</code></p>
  
  <pre><code>version: '2'
  
  services:
    db:
      image: postgres
    tox:
      build: .
      depends_on:
        - db
      volumes:
        - ".:/src:ro"
  </code></pre>
  
  <p>My <code>tox.ini</code></p>
  
  <pre><code>[tox]
  envlist = {py27}-{django18,django19,django110,django111},{py35}-{django18,django19,django110,django111,django20}
  skipsdist = {env:TOXBUILD:false}
  
  [testenv]
  sitepackages = False
  deps=
      pytest==2.9.2
      pytest-capturelog==0.7
      pytest-django==2.9.1
      psycopg2==2.7.3.2
      pytest-pep8==1.0.6
      freezegun==0.3.9
      pytz==2017.3
  
      django18: Django&gt;=1.8,&lt;1.9
      django19: Django&gt;=1.9,&lt;1.10
      django110: Django&gt;=1.10,&lt;1.11
      django111: Django&gt;=1.11,&lt;2.0
      django20: Django&gt;=2.0,&lt;2.1
  commands = py.test tests {posargs}
  </code></pre>
  
  <p>My <code>pytest.ini</code></p>
  
  <pre><code>[pytest]
  addopts=-l -q --capture=no
  pep8ignore = E501
  norecursedirs = .robe .idea
  python_files = tests.py test_*.py
  DJANGO_SETTINGS_MODULE = test_app.settings
  </code></pre>
  
  <p>My <code>dockerfile</code>:</p>
  
  <pre><code>FROM themattrix/tox
  </code></pre>