• Refactor do_ci.sh to split the cmake and environment setup from the
  build and test. This makes it convenient for devs to invoke arbitrary
  make targets in the Docker image, e.g.

  docker run -t -i -v $PWD:/source lyft/envoy-build:latest /bin/bash -c
  "cd /source && . ./ci/build_setup.sh && make fix_format"

• Add EXTRA_TEST_ARGS env var to pass addition flags to envoy-test. This
  is useful, for example, when applying a --gtest_filter to restrict the
  test run to only failing tests.

• Add a do_ci.sh fix_format target.

• Add gdb, strace to the image and a RUN_TEST_UNDER environment variable
  that can be used to have envoy-test run under these programs.

• UNIT_TEST_ONLY env var that will skip the non-envoy-test aspects of
  the test process.

Example command for debugging only tests matching Dns:

docker run -t -i -v $PWD:/source lyft/envoy-build:latest /bin/bash -c
"cd /source && UNIT_TEST_ONLY=1 RUN_TEST_UNDER='gdb --args' ./ci/do_ci.sh debug
--gtest_filter='Dns'