Repin NNG refs to capture proxy reconnect fix

A fix to address NNG issues when reconnecting a session
through a proxy was made on August 9, 2019 and merged into
the NNG master branch with commit ID e618abf8f3db2a9. This
change repins the references to the NNG repo such that the
fix is incorporated in the NNG library.

Signed-off-by: E. Scott Daniels <daniels@research.att.com>
Change-Id: I84d707b6ac65af8e7d78fa6814208532603ea177
diff --git a/docker/bldr-ubuntu16-c-go/Dockerfile b/docker/bldr-ubuntu16-c-go/Dockerfile
index 0212d7d..60007c4 100644
--- a/docker/bldr-ubuntu16-c-go/Dockerfile
+++ b/docker/bldr-ubuntu16-c-go/Dockerfile
@@ -15,13 +15,6 @@
 # limitations under the License.
 
 # Builder image with superset of CI tools for RIC components
-# Uses Ubuntu 16.04 base then adds:
-# Standard apt packages including gcc and g++ 5.4
-# Cmake 3.14.5 from binary distro to /usr/local/bin
-# Golang 1.12 from binary distro to /usr/local/go
-# Ninja 1.9 from binary distro to /usr/local/bin
-# Nanomsg Next Gen v1.1.1 from source distro
-# Boost 1.69 from source distro
 
 FROM nexus3.o-ran-sc.org:10001/ubuntu:16.04
 
@@ -64,8 +57,12 @@
     && chmod +x ninja \
     && mv ninja /usr/local/bin
 
-RUN git clone --branch v1.1.1 https://github.com/nanomsg/nng.git \
+# NNG repo is not frequently tagged so it's pinned to a commit hash.
+# This commit provides fix to the proxy-reconnect
+# bug that we identified:  https://github.com/nanomsg/nng/issues/970
+RUN git clone https://github.com/nanomsg/nng.git \
     && cd nng \
+    && git checkout e618abf8f3db2a94269a79c8901a51148d48fcc2 \
     && mkdir build \
     && cd build \
     && cmake -DBUILD_SHARED_LIBS=1 -G Ninja .. \