fix Dockerfile

[CaoShuFeng]

Without this change, make docker-image would fail:

    build-base (missing):
ERROR: unsatisfiable constraints:
    required by: world[build-base]
The command '/bin/sh -c mkdir -p /usr/local/src &&     cd /usr/local/src &&     curl -L https://github.com/gohugoio/hugo/releases/download/v${HUGO_VERSION}/hugo_extended_${HUGO_VERSION}_linux-64bit.tar.gz | tar -xz &&     apk add build-base &&     apk add libc6-compat &&     mv hugo /usr/local/bin/hugo &&     curl -L https://bin.equinox.io/c/dhgbqpS8Bvy/minify-stable-linux-amd64.tgz | tar -xz &&     mv minify /usr/local/bin &&     addgroup -Sg 1000 hugo &&     adduser -Sg hugo -u 1000 -h /src hugo' returned a non-zero code: 1

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Please delete this note before submitting the pull request.
For 1.12 Features: set Milestone to 1.12 and Base Branch to release-1.12
Help editing and submitting pull requests: https://deploy-preview-9510--kubernetes-io-master-staging.netlify.com/docs/contribute/start/#submit-a-pull-request.
Help choosing which branch to use, see
https://kubernetes.io/docs/contribute/start#choose-which-git-branch-to-use.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[k8sio-netlify-preview-bot]

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

Built with commit 1928906

https://deploy-preview-10312--kubernetes-io-master-staging.netlify.com

[k8sio-netlify-preview-bot]

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

Built with commit 1928906

https://deploy-preview-10312--kubernetes-io-master-staging.netlify.com

[Bradamant3]

/assign @lucperkins

[lucperkins]

/lgtm
/approve

[lucperkins]

/assign @chenopis

[Bradamant3]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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 [Bradamant3]

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