Bazel: modified licenses.tar file after running acceptance test

[sgmonroy]

When running acceptance tests on Master, the following files are modified:
docker/perapp/licenses.tar

Current Master commit hash:
commit 9a8e66f

[worxli]

More precisely the file is modified when running make -C docker/perapp.

[kormat]

This problem still exists, reopening.

[kormat]

From an off-line discussion with @sustrik:

• Python packages from pip come with their own licenses.
• Apt packages put their licesnses into /usr/share/doc/, and docker/perapp/files/copy_package already handles this

So the remaining thing we need to care about is licenses from go libraries we depend on.

Proposed fix:

• Add a go_deps target to the top-level Makefile. It should run tools/fetch.sh if WORKSPACE has changed, and generate a BUILD.bzl file containing a target that depends on all go dependencies.
• Convert docker/perapp/licenses.tar into a bazel target, that depends on the target generated by tools/fetch.sh
• Have all docker image targets depend on licenses.tar

[scrye]

Licensing changed in #3692, closing this as it should now be fixed.