Add Docker setup for running tools/format.sh. (#299)

tools/docker-format/Dockerfile

@@ -0,0 +1,8 @@
+FROM ubuntu:cosmic
+
+RUN apt update && \
+    apt install -y clang-format golang git python-pip && \
+    go get -v github.com/bazelbuild/buildtools/buildifier && \
+    pip install cmake_format
+
+CMD ["/bin/bash"]

tools/docker-format/README.md

@@ -0,0 +1,9 @@
+Docker container for automatic formatting.
+
+Usage:
+
+```shell
+cd opencensus-cpp
+docker build --tag opencensus-cpp/format tools/docker-format
+docker run -v $PWD:/opencensus-cpp -it opencensus-cpp/format /opencensus-cpp/tools/docker-format/run.sh
+```

tools/docker-format/run.sh

@@ -0,0 +1,4 @@
+#!/bin/bash
+export PATH=/root/go/bin:$PATH
+cd /opencensus-cpp
+tools/format.sh