add git-credential-env

and configure git to use it by default

`git-credential-env` only echoes the $GIT_CREDENTIAL_ENV environment variable
when git asks for https credentials.

with this, setting GIT_CREDENTIAL_ENV="username=name\npassword=xxx" sets the credentials
that will be used.

e.g.

    docker run -e GIT_CREDENTIAL_ENV="username=name\npassword=xxx" jupyter/repo2docker https://github.com/private/repo

Dockerfile

@@ -5,5 +5,6 @@ RUN apk add --no-cache git
 
 RUN mkdir /tmp/src
 ADD . /tmp/src
-RUN pip3 install --no-cache-dir /tmp/src
-
+RUN pip3 install --no-cache-dir /tmp/src && \
+    cp /tmp/src/docker/git-credential-env /usr/local/bin/git-credential-env && \
+    git config --system credential.helper env

docker/git-credential-env

@@ -0,0 +1,2 @@
+#!/bin/sh
+echo -e $GIT_CREDENTIAL_ENV