[v1.x] Fix CD builds - error in docker command.

[josephevans]

Description

Remove extra --build-arg causing docker command to fail.

Errors with:

15:53:16 docker build -t mxnet/python:nightly_cpu_py3 --build-arg --build-arg BASE_IMAGE=ubuntu:16.04 --build-arg MXNET_COMMIT_ID=xxx -f cd/python/docker/Dockerfile ./wheel_build
15:53:16 "docker build" requires exactly 1 argument.
15:53:16 See 'docker build --help'.

This may not of been an issue, but we recently updated docker versions and this command is failiing.

Checklist

Essentials

• PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented

Changes

• Feature1, tests, (and when applicable, API doc)
• Feature2, tests, (and when applicable, API doc)

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here

[mxnet-bot]

Hey @josephevans , Thanks for submitting the PR
All tests are already queued to run once. If tests fail, you can trigger one or more tests again with the following commands:

• To trigger all jobs: @mxnet-bot run ci [all]
• To trigger specific jobs: @mxnet-bot run ci [job1, job2]

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