Produce docker autoconfig logs (#1) (#223)

* Produce docker autoconfig logs (#1)

Docker autoconfig is not printing out anywhere the results of executing commands inside the container. This PR adds a new output to the rule that produces the log.
Example outputs produced by an autoconfig rule:
Target //tests/config:ubuntu-xenial-autoconfig up-to-date:
bazel-bin/tests/config/ubuntu-xenial-autoconfig-layer.tar
bazel-bin/tests/config/ubuntu-xenial-autoconfig.tar
bazel-bin/tests/config/ubuntu-xenial-autoconfig.digest
bazel-bin/tests/config/ubuntu-xenial-autoconfig_log.log
bazel-bin/tests/config/ubuntu-xenial-autoconfig_outputs.tar

* produce message to debug errors

* resolved review comments
* added print message in rule that shows instructions for reproducing
execution of docker container. Rule prints out (example):

== Docker autoconfig will run. ==
To debug any errors run:
> docker run -d <image_id> bash
Where <image_id> is the image id printed out by the ubuntu-xenial-autoconfig_extract.tar rule.
Then run:
>/ubuntu-xenial-autoconfig_install.sh
from inside the container.