Typo in Dockerfile causes ARCH to be ignored during build process

[slapula]

What this PR does / why we need it:

arm64 support was recently added in #680 however there was a typo in the Dockerfile that sneaked through during the review process. This PR fixes that typo. I have already tested this in my own arm64 environment and was able to produce working images.

Which issue(s) this PR fixes:

Fixes #803

Does this PR introduce a user-facing change?

NONE

Additional Notes for your reviewer:

Review Checklist:

• Follows the developer guidelines
• Relevant tests are added or updated
• Relevant docs in this repo added or updated
• Relevant carvel.dev docs added or updated in a separate PR and there's
  a link to that PR
• Code is at least as readable and maintainable as it was before this
  change

Additional documentation e.g., Proposal, usage docs, etc.:

[vmwclabot]

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[benmoss]

Thanks for filing this! Are you able to sign the CLA?

[slapula]

@benmoss Yup, I have signed it and received the confirmation email.

[vmwclabot]

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