Optionally clone Morpheus from a URL during Docker builds

[dagardner-nv]

Description

• Optionally clone from a git url if MORPHEUS_GIT_URL & MORPHEUS_GIT_TAG are defined, otherwise the host repo is used.
• Fixes issue when morpheus is in a submodule and the parent repo is not accessible to the Docker workspace
• Clone is now performed in a new shell script docker/clone_source.sh

Closes #1870

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• I am familiar with the Contributing Guidelines.
• When the PR is ready for review, new or existing tests cover these changes.
• When the PR is ready for review, the documentation is up to date with these changes.