Prevent to send all files to docker daemon

[superbrothers]

This PR prevents to send all files to docker daemon in order to build kubernetes-hugo image.

Currently, all files in this repository are sent to docker daemon when building kubernetes-hugo image. But they are not used in Dockerfile, so with this PR, I adds .dockerignore file to prevent to send all files to docker daemon.

Before

$ make docker-image
docker build . --tag kubernetes-hugo --build-arg HUGO_VERSION=0.47.1
Sending build context to Docker daemon    229MB
Step 1/7 : FROM alpine:latest
...

After

$ make docker-image
docker build . --tag kubernetes-hugo --build-arg HUGO_VERSION=0.47.1
Sending build context to Docker daemon  3.584kB
Step 1/7 : FROM alpine:latest
...

[k8sio-netlify-preview-bot]

Deploy preview for kubernetes-io-master-staging ready!

Built with commit 166556d

https://deploy-preview-10129--kubernetes-io-master-staging.netlify.com

[Bradamant3]

/assign @lucperkins

[lucperkins]

/lgtm
/approve

[superbrothers]

/assign @Bradamant3

[Bradamant3]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Bradamant3, lucperkins

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