Fill in testing Dockerfile stub.

blimp/engine/testing/Dockerfile

-# Testing dockerfile stub
-# TODO(jessicag): Fill in (crbug.com/616945).
+# This Dockerfile is used to build a filesystem environment containing
+# binaries and required files for Blimp Engine test and tests for dependencies
+# of Blimp Engine. It is built on the same base image that is used to run the
+# Engine itself.
+FROM base:latest
+
+RUN mkdir -p /out/test/
+
+# The glob below expands to all files, but does not add directories
+# recursively.
+# Test binaries assume src directory is 2 levels down. While this behavior can
+# be changed with flags, the directory structure is set up to minimize
+# potential problems with initial integration.
+ADD * /out/test/
+
+RUN chown -R blimp_user /out/test/
+
+USER blimp_user