container up - skip ACR build if the docker hub image name is specified

[nking-1]

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Fixes a bug where the ACR code upload and build step happens even if you specify an image to be pulled from DockerHub.

This checklist is used to make sure that common guidelines for a pull request are followed.

General Guidelines

• Have you run azdev style <YOUR_EXT> locally? (pip install azdev required)
• Have you run python scripts/ci/test_index.py -q locally?

For new extensions:

• My extension description/summary conforms to the Extension Summary Guidelines.

[azuresdkci]

If this PR is for a new extension or change to an existing extension, use the following to try out the changes in this PR:

docker run -it microsoft/azure-cli:latest
export EXT=<NAME>
pip install --upgrade --target ~/.azure/cliextensions/$EXT "git+https://github.com/Nking92/azure-cli-extensions.git@master#subdirectory=src/$EXT&egg=$EXT"