Update the build image to buster

[markmandel]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line:

/kind breaking
/kind bug

/kind cleanup

/kind documentation
/kind feature
/kind hotfix

What this PR does / Why we need it:
Updating old debian images 👍

Which issue(s) this PR fixes:
Work on #1488

Special notes for your reviewer:
None.

[agones-bot]

Build Succeeded 👏

Build Id: 9e5b1a49-a3cb-4e03-b146-f9c11d94eb40

The following development artifacts have been built, and will exist for the next 30 days:

• image: gcr.io/agones-images/agones-controller:1.7.0-d76c9b8
• image: gcr.io/agones-images/agones-sdk:1.7.0-d76c9b8
• image: gcr.io/agones-images/agones-ping:1.7.0-d76c9b8
• Linux C++ SDK (build): agonessdk-1.7.0-d76c9b8-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.7.0-d76c9b8.zip

A preview of the website (the last 30 builds are retained):

• https://d76c9b8-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/GoogleCloudPlatform/agones.git pull/1608/head:pr_1608 && git checkout pr_1608
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.7.0-d76c9b8

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: EricFortin, markmandel

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• OWNERS [EricFortin,markmandel]

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