protoc-artifacts: Avoid checking out protobuf code

It is a bad idea to check out code into the docker image, as it will be
out-of-date. It is better to have the image just be the environment, and
let any scripts that need source check them out themselves.

This fixes #4419 in that it allows the image to build again, albeit
users would need to use wget to grab the source of the version of
protobuf they wish.

protoc-artifacts/Dockerfile

@@ -35,7 +35,5 @@ RUN rpm --rebuilddb && yum install -y devtoolset-1.1 \
                    devtoolset-1.1-libstdc++-devel \
                    devtoolset-1.1-libstdc++-devel.i686
 
-RUN git clone --depth 1 https://github.com/google/protobuf.git
-
 # Start in devtoolset environment that uses GCC 4.7
 CMD ["scl", "enable", "devtoolset-1.1", "bash"]

protoc-artifacts/README.md

@@ -143,7 +143,11 @@ To run the image:
 $ docker run -it --rm=true protoc-artifacts
 ```
 
-The Protobuf repository has been cloned into ``/protobuf``.
+To checkout protobuf (run within the container):
+```
+$ # Replace v3.5.1 with the version you want
+$ wget -O - https://github.com/google/protobuf/archive/v3.5.1.tar.gz | tar xvzp
+```
 
 ### Tips for deploying on Windows
 Under Windows the following error may occur: ``gpg: cannot open tty `no tty':