Dockerfile: simplify procobuf install

Use unzip options to unpack directly to /usr/local/{include,bin},
to avoid unnecessary I/O.

Unfortunately unzip does not support unpacking from a pipe (due
to a limitation of .zip format -- the index is at EOF) so we still
have to save the .zip to disk, read it back, and remove. If they
could only provide tarballs... *sigh*

Signed-off-by: Kir Kolyshkin <[email protected]>

Dockerfile

@@ -4,16 +4,12 @@ FROM golang:1.11.0-stretch
 RUN apt-get update && apt-get install -y make git unzip
 
 # should stay consistent with the version we use in Circle builds
-ARG PROTOC_VERSION=3.5.0
-# make a directory to do these operations in
-RUN export PROTOC_TMP_DIR=protoc && mkdir -p $PROTOC_TMP_DIR && cd $PROTOC_TMP_DIR \
-  # download the pre-built protoc binary
-  && curl --silent --show-error --location --output protoc.zip \
-    https://github.com/google/protobuf/releases/download/v$PROTOC_VERSION/protoc-$PROTOC_VERSION-linux-x86_64.zip \
-  # move the binary to /bin. move the well-known types ot /usr/local/include
-  && unzip protoc.zip && mv bin/protoc /bin/protoc && mv include/* /usr/local/include \
-  # remove all of the installation files
-  && cd .. && rm -rf $PROTOC_TMP_DIR
+ARG PROTOC_VERSION=3.6.1
+# download and install protoc binary and .proto files
+RUN curl --silent --show-error --location --output protoc.zip \
+  https://github.com/google/protobuf/releases/download/v$PROTOC_VERSION/protoc-$PROTOC_VERSION-linux-x86_64.zip \
+  && unzip -d /usr/local protoc.zip include/\* bin\/* \
+  && rm -f protoc.zip
 
 WORKDIR /go/src/github.com/docker/swarmkit/