chore: Added Dockerfiles to .dockerignore

[hbelmiro]

Since we're copying all files into the images, when we change anything in a Dockerfile, the container engine invalidates the cached layer at the COPY . . point. That makes subsequent builds unnecessarily long having to copy and build everything again, even when no source code was changed.

This PR adds Dockerfiles to .dockerignore so they are ignored when running COPY . .. That way we leverage the existing caches when no source code was changed.

Checklist:

• You have signed off your commits
• The title for your pull request (PR) should follow our title convention. Learn more about the pull request title convention used in this repository.

[google-oss-prow]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[rimolive]

/lgtm

[HumairAK]

For the curious on docs for this:

You can use the .dockerignore file to exclude the Dockerfile and .dockerignore files. These files are still sent to the builder as they're needed for running the build. But you can't copy the files into the image using ADD, COPY, or bind mounts. [1]

Since we don't keep any Dockerifles in the root, and all builds specify via -f, this should, I think, be a fairly safe change.

/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: franciscojavierarceo, HumairAK

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [HumairAK]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment