move dockerfile appendix into LegacyBinderDockerBuildPack.dockerfile_appendix

and make it overrideable

repo2docker/detectors.py

@@ -1,6 +1,7 @@
 import os
 import sys
 import subprocess
+from textwrap import dedent
 
 import docker
 from docker.utils import kwargs_from_env
@@ -50,26 +51,27 @@ class DockerBuildPack(BuildPack):
                 else:
                     sys.stdout.write(progress['stream'])
 
-_legacy_binder_docker_appendix = """
-USER root
-COPY . /home/main/notebooks
-RUN chown -R main:main /home/main/notebooks
-USER main
-WORKDIR /home/main/notebooks
-ENV PATH /home/main/anaconda2/envs/python3/bin:$PATH
-RUN conda install -n python3 notebook==5.0.0 ipykernel==4.6.0 && \
-    pip install jupyterhub==0.7.2 && \
-    conda remove -n python3 nb_conda_kernels && \
-    conda install -n root ipykernel==4.6.0 && \
-    /home/main/anaconda2/envs/python3/bin/ipython kernel install --sys-prefix && \
-    /home/main/anaconda2/bin/ipython kernel install --prefix=/home/main/anaconda2/envs/python3
-ENV JUPYTER_PATH /home/main/anaconda2/share/jupyter:$JUPYTER_PATH
-CMD jupyter notebook --ip 0.0.0.0
-"""
 
 class LegacyBinderDockerBuildPack(DockerBuildPack):
     
     name = Unicode('Legacy Binder Dockerfile')
+    dockerfile_appendix = Unicode(dedent(r"""
+    USER root
+    COPY . /home/main/notebooks
+    RUN chown -R main:main /home/main/notebooks
+    USER main
+    WORKDIR /home/main/notebooks
+    ENV PATH /home/main/anaconda2/envs/python3/bin:$PATH
+    RUN conda install -n python3 notebook==5.0.0 ipykernel==4.6.0 && \
+        pip install jupyterhub==0.7.2 && \
+        conda remove -n python3 nb_conda_kernels && \
+        conda install -n root ipykernel==4.6.0 && \
+        /home/main/anaconda2/envs/python3/bin/ipython kernel install --sys-prefix && \
+        /home/main/anaconda2/bin/ipython kernel install --prefix=/home/main/anaconda2/envs/python3
+    ENV JUPYTER_PATH /home/main/anaconda2/share/jupyter:$JUPYTER_PATH
+    CMD jupyter notebook --ip 0.0.0.0
+    """), config=True)
+    
     def detect(self, workdir):
         dockerfile = os.path.join(workdir, 'Dockerfile')
         if not os.path.exists(dockerfile):
@@ -86,8 +88,9 @@ class LegacyBinderDockerBuildPack(DockerBuildPack):
         return False
 
     def amend_dockerfile(self, dockerfile):
+        print(self.dockerfile_appendix)
         with open(dockerfile, 'a') as f:
-            f.write(_legacy_binder_docker_appendix)
+            f.write(self.dockerfile_appendix)
 
 
 class S2IBuildPack(BuildPack):