message: docker: Support `ENV`.
diff: @@ -45,7 +45,7 @@ class DockerFile(drake.Node):
  self.__cmd = []
  self.__entry_point = []
  self.__ports = []
  - self.__runs = []
  + self.__steps = []
  self.__volumes = []
  DockerFile.Builder(self)
  @@ -81,7 +81,10 @@ class DockerFile(drake.Node):
  self.__volumes = volumes
  def run(self, cmd):
  - self.__runs.append(cmd)
  + self.__steps.append(('RUN', cmd))
  +
  + def env(self, k, v):
  + self.__steps.append(('ENV', '{} {}'.format(k, v)))
  @property
  def image(self):
  @@ -112,8 +115,8 @@ class DockerFile(drake.Node):
  return self.__ports
  @property
  - def runs(self):
  - return self.__runs
  + def steps(self):
  + return self.__steps
  @property
  def volumes_(self):
  @@ -128,7 +131,7 @@ class DockerFile(drake.Node):
  'labels': self.__labels,
  'maintainer': self.__maintainer,
  'ports': self.__ports,
  - 'runs': self.__runs,
  + 'steps': self.__steps,
  'volumes': self.__volumes,
  }
  @@ -148,8 +151,8 @@ class DockerFile(drake.Node):
  file = f)
  for k, v in self.__dockerfile.labels.items():
  print('LABEL %s="%s"' % (k, v), file = f)
  - for run in self.__dockerfile.runs:
  - print('RUN %s' % run, file = f)
  + for cmd, args in self.__dockerfile.steps:
  + print('{} {}'.format(cmd, args), file = f)
  for p, nodes in self.__dockerfile._DockerFile__adds.items():
  for add in rootify(
  chain(*(parents(n.path().without_prefix(root))
  