Once and for all deal with dockerpy's brokenness

layer.yaml

@@ -1,4 +1,5 @@
 includes: ['layer:basic']
+tactics: ['tactics.docker.DockerWheelhouseTactic']
 repo: https://github.com/juju-solutions/layer-docker.git
 defines:
   skip-install:

tactics/docker.py

@@ -0,0 +1,28 @@
+from charmtools.build.tactics import WheelhouseTactic
+from charmtools.utils import Process
+import tempfile
+from path import Path as path
+
+
+class DockerWheelhouseTactic(WheelhouseTactic):
+    ''' The references to process, path, and tempfile are implicit
+    below because  they come from the global scope of
+    charmtools.utils which gets passed in during
+    charmtools.utils.load_class
+    Investigate there if there is abrupt breakage here.
+    '''
+    def __call__(self, venv=None):
+        create_venv = venv is None
+        venv = venv or path(tempfile.mkdtemp())
+        pip = venv / 'bin' / 'pip3'
+        wheelhouse = self.target.directory / 'wheelhouse'
+        wheelhouse.mkdir_p()
+        if create_venv:
+            Process(('virtualenv', '--python', 'python3', venv)).exit_on_error()()  # noqa
+            Process((pip, 'install', '-U', 'pip', 'wheel')).exit_on_error()()  # noqa
+        Process((pip, 'install', '-U', 'setuptools')).exit_on_error()()  # noqa
+        for tactic in self.previous:
+            tactic(venv)
+        self._add(pip, wheelhouse, '-r', self.entity)
+        if create_venv:
+            venv.rmtree_p()

wheelhouse.txt

@@ -1,4 +1,5 @@
+setuptools>=19.6,<=20.0
 charms.docker>=0.0.1,<=2.0.0
-docker-py==1.7.2
-docker-compose>=1.0.0,<=2.0.0
 vcversioner>=2.0.0,<=3.0.0
+docker-compose>=1.0.0,<=2.0.0
+