Fix dockerfile builds and add tests for them

.travis.yml

@@ -18,3 +18,4 @@ env:
   - REPO_TYPE=conda
   - REPO_TYPE=venv
   - REPO_TYPE=julia
+  - REPO_TYPE=dockerfile

repo2docker/detectors.py

@@ -677,6 +677,15 @@ class DockerBuildPack(BuildPack):
         with open('Dockerfile') as f:
             return f.read()
 
+    def build(self, image_spec):
+        client = docker.APIClient(version='auto', **docker.utils.kwargs_from_env())
+        for line in client.build(
+                path=os.getcwd(),
+                tag=image_spec,
+                decode=True
+        ):
+            yield line
+
 class LegacyBinderDockerBuildPack(BuildPack):
 
     name = 'Legacy Binder Dockerfile'

tests/dockerfile/jupyter-stack/Dockerfile

@@ -0,0 +1,4 @@
+FROM jupyter/base-notebook:b4dd11e16ae4
+
+RUN pip install there
+ADD verify verify

tests/dockerfile/jupyter-stack/verify

@@ -0,0 +1,7 @@
+#!/usr/bin/env python
+import sys
+
+assert sys.version_info[:2] == (3, 6)
+
+import jupyter
+import there

tests/dockerfile/simple/Dockerfile

@@ -0,0 +1,6 @@
+FROM alpine:3.5
+
+ENTRYPOINT "/bin/sh"
+
+ADD sayhi.sh /usr/local/bin/sayhi.sh
+ADD verify verify

tests/dockerfile/simple/sayhi.sh

@@ -0,0 +1,3 @@
+#!/bin/bash
+echo hi
+exit 0

tests/dockerfile/simple/verify

@@ -0,0 +1,3 @@
+#!/bin/sh
+set -e
+/usr/local/bin/sayhi.sh