Support Dockerfile.dockerignore

[agrinh]

Fixes: Did not create an issue for this, please let me know if I should.

Description
Add support for getting dependencies from Dockerfile.dockerignore files. See e.g. moby/buildkit#901 . Since the same code produces dependencies for Kaniko I expect this will also make Skaffold align better with Kaniko (GoogleContainerTools/kaniko#801).

User facing changes
Users with multiple Dockerfiles will now be able to have different .dockerignore files. For example, this is the behavior with the following context:

.
├── .dockerignore
├── Dockerfile
├── Dockerfile.dev
└── Dockerfile.dev.dockerignore

before: docker would respect Dockerfile.dev.dockerignore but skaffold would not take it into account when producing dependencies, and syncing could break e.g. if .dockerignore hid files used by Dockerfile.dev.dockerignore, leading to unexpected behavior.

after: skaffold will make Dockerfile.dev.dockerignore take precedence over .dockerignore (will fall back to .dockerignore if it doesn't exist).

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[agrinh]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[dgageot]

Overall, it's great. A few things can be improved.

[agrinh]

Thanks for the suggestions @dgageot, fixed your comments.

[codecov]

Codecov Report

Merging #3837 into master will decrease coverage by <.01%.
The diff coverage is 76.47%.

Impacted Files	Coverage Δ
pkg/skaffold/docker/syncmap.go	67.16% <100%> (ø)	⬆️
pkg/skaffold/docker/dependencies.go	75% <75%> (+1.08%)	⬆️
...affold/kubernetes/portforward/kubectl_forwarder.go	65.85% <0%> (-2.44%)	⬇️
pkg/skaffold/debug/transform_nodejs.go	77.87% <0%> (-0.77%)	⬇️
pkg/skaffold/docker/docker_init.go	100% <0%> (ø)	⬆️
pkg/skaffold/initializer/build/util.go	100% <0%> (ø)	⬆️
pkg/skaffold/initializer/build/builders.go	57.69% <0%> (ø)	⬆️

[agrinh]

Weird, integration tests failed at first but they succeeded after restarting.