Add missing license headers.

[ssbr]

From https://opensource.google/docs/releasing/preparing/ :

BEST PRACTICE: Add license headers to any files that can be added to (i.e. anything that takes the format of a source file and supports file comments). This would include things like YAML files.

I believe this very directly applies to .travis.yml, and reasonably also applies to Dockerfile. Wasn't sure what year to put, just copy-pasted the license header from an existing Go source file and changed the commenting syntax.

(BTW, I mentioned this in the relevant (but internal to Google) b/165621580.)

[willnorris]

Hey @ssbr, now that .travis.yml no longer exists, but there are now a handful of new YAML files, you wanna run this again against the current master branch?

[ssbr]

I think that's done now. Any other files you have in mind?

---

It's been a while so I didn't remember how to run addlicense and had some trouble there. :) Apparently, one must run go get addlicense in a different directory than the git repo you're changing, and then cd into the git repo. If you don't, it updates the go.mod and go.sum files for some reason. (I am sure Go experts are rolling their eyes right now.)

[willnorris]

I guess if you really wanna cover everything, then the .github/workflow/** files also?

It seems a little much to have license headers in some of these files, but to your point that is Google's open source guidance. I believe Google will begin accepting SPDX style headers reasonably soon, so that will reduce these down to just two lines.

btw, you can also just run it as:

go run . -ignore "testdata/**" .