Add license to image and gemspec

[Nishnha]

This adds the LICENSE to the home directory of the Docker image.

The dependabot-* gems on rubygems currently have the license set as 'nonstandard' and do not include a license file.
TBD: rake gems:release will not include the LICENSE in the dependabot-* gems in this PR.

[Nishnha]

So the license we use is actually not on the list of common open-source licenses that rubygems recognizes. There is a PR to add v3 but it has not been merged: spdx/license-list-XML#960

We'll have to list the license as "Nonstandard" like it was before and include the LICENSE file in the top-level directory of the gem so that it can get picked up by licensed

[Nishnha]

The scope of this PR has been reduced to only including the LICENSE file in the Dependabot-Core Docker image. We will punt on adding the LICENSE to the dependabot-* Gems themselves since it's non-trivial with this Nonstandard license.

Since the LICENSE will not be included in the Gems, we will also have to omit the dependabot-* Gems from being vendored with licensed, but that isn't that big of an issue since Core is owned by us and anyone can look at our required Gems for licensing info.