Support setting appendix on Dockerfiles too

Until now, 'appendix' was ignored for repos built with
their own Dockerfile. This fixes that, and sets the appendix
on repos built with Dockerfiles too.

I want to set a REPO_URL env var for all images built with
repo2docker-action
(jupyterhub/repo2docker-action#86)
and this is needed for that.

repo2docker/buildpacks/docker.py

@@ -1,6 +1,7 @@
 """Generates a variety of Dockerfiles based on an input matrix
 """
 import os
+import tempfile
 import docker
 from .base import BuildPack
 
@@ -15,10 +16,21 @@ def detect(self):
         return os.path.exists(self.binder_path("Dockerfile"))
 
     def render(self, build_args=None):
-        """Render the Dockerfile using by reading it from the source repo"""
+        """
+        Render the Dockerfile used by reading it from the source repo
+
+        If an appendix is configured, it will be appended to the Dockerfile
+        found in the source repo.
+        """
         Dockerfile = self.binder_path("Dockerfile")
         with open(Dockerfile) as f:
-            return f.read()
+            content = f.read()
+
+        if self.appendix:
+            content += "\n"
+            content += self.appendix
+
+        return content
 
     def build(
         self,
@@ -45,17 +57,20 @@ def build(
             # we use no swap.
             limits = {"memory": memory_limit, "memswap": memory_limit}
 
-        build_kwargs = dict(
-            path=os.getcwd(),
-            dockerfile=self.binder_path(self.dockerfile),
-            tag=image_spec,
-            buildargs=build_args,
-            container_limits=limits,
-            cache_from=cache_from,
-            labels=self.get_labels(),
-        )
+        with tempfile.NamedTemporaryFile(mode="w") as f:
+            f.write(self.render())
+            f.flush()
+            build_kwargs = dict(
+                path=os.getcwd(),
+                dockerfile=f.name,
+                tag=image_spec,
+                buildargs=build_args,
+                container_limits=limits,
+                cache_from=cache_from,
+                labels=self.get_labels(),
+            )
 
-        build_kwargs.update(extra_build_kwargs)
+            build_kwargs.update(extra_build_kwargs)
 
-        for line in client.build(**build_kwargs):
-            yield line
+            for line in client.build(**build_kwargs):
+                yield line

tests/dockerfile/simple/verify

@@ -1,3 +1,6 @@
 #!/bin/bash
 set -euo pipefail
 /usr/local/bin/sayhi.sh
+
+# Test that the appendix worked, even though this is a custom Dockerfile
+test $(cat /tmp/appendix) == "appendix"