Add env with required path for appmesh integration.

[fierlion]

Summary

Pulling this change forward:
Newer version of Docker mounts tini to /sbin/init which conflicts our existing /sbin:/sbin mount. Updating the mount to /sbin:/host/sbin and adds /host/sbin to the PATH env (needed by the appmesh plugin to find the iptable binary) fixes the problem.
0a5c60c

Implementation details

Added the path to the agent-config.json to build into the ECS Agent container image.

Testing

built the ecs-agent image using make dockerfree-agent-image
loaded the new image with docker load < docker load < ecs-agent-v1.62.2.tar
docker inspect <loaded image ID> shows:

        "Config": {
            ...
            "Env": [
                "PATH=/host/sbin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
            ],
            ...

New tests cover the changes: no

Description for the changelog

Add appmesh path to agent container image config.

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.