BF: fix use of env_vars.sh for docker container

Path of env_vars.sh file should be relative to the repository, but when
we run in docker, the PWD is /, not /io, which is the repository root.

README.rst

@@ -322,15 +322,20 @@ To use these scripts
 
   Optionally you can specify a different location for ``config.sh`` file with
   the ``$CONFIG_PATH`` environment variable.
-  
+
 * Optionally, create an ``env_vars.sh`` file to override the defaults for any
-  environment variables used by ``configure_build.sh``/``library_builders.sh``.
-  In Linux, they cannot be just set in the initial environment because the
-  build runs in Docker, so only the variables explicitly passed to
-  ``docker run`` are propagated.
-  
-  Likewise, you can specify a different location for the file by setting the
-  the ``$ENV_VARS_PATH`` environment variable.
+  environment variables used by
+  ``configure_build.sh``/``library_builders.sh``. In Linux, the environment
+  variables used for the build cannot be set in the ``.travis.yml`` file,
+  because the build processing runs in a Docker container, so only the only
+  environment variables that reach the container are those passed in via the
+  ``docker run`` command, or those set in ``env_vars.sh``.
+
+  As for the ``config.sh`` file, you can specify a different location for the
+  file by setting the ``$ENV_VARS_PATH`` environment variable.  The path in
+  ``$ENV_VARS_PATH`` is relative to the repository root directory.  For
+  example, if your repository had a subdirectory ``scripts`` with a file
+  ``my_env_vars.sh``, you should set ``ENV_VARS_PATH=scripts/my_env_vars.sh``.
 
 * Make sure your project is set up to build on travis-ci, and you should now
   be ready (to begin the long slow debugging process, probably).

docker_build_wrap.sh

@@ -17,7 +17,8 @@ WHEEL_SDIR=${WHEEL_SDIR:-wheelhouse}
 # Location of `config.sh` file, default "./config.sh"
 CONFIG_PATH=${CONFIG_PATH:-config.sh}
 
-ENV_VARS_PATH=${ENV_VARS_PATH:-env_vars.sh}
+# Path is relative to repository from which we ran
+ENV_VARS_PATH="/io/${ENV_VARS_PATH:-env_vars.sh}"
 
 # Always pull in common and library builder utils
 MULTIBUILD_DIR=$(dirname "${BASH_SOURCE[0]}")