[0.6.0] Fixes #897 -- --path flag inconsistencies for porter update build

[abelanger5]

Pull request type

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• Bugfix
• Feature
• Other (please describe):

Pull request checklist

Please check if your PR fulfills the following requirements:

• If it's a backend change, tests for the changes have been added and go test ./... runs successfully from the root folder.
• If it's a frontend change, Prettier has been run
• Docs have been reviewed and added / updated if needed

What is the current behavior?

Issue Number: #897

Is really three issues:

• --path flag isn't used when --source github, although it should be used to specify the relative path from the repository root to the build context
• --dockerfile flag isn't used when --source github, although it should be used to specify the relative path from the repository root to the dockerfile
• the --dockerfile path isn't added to the Docker build context if it's outside of the build context

Technical Spec/Implementation Notes

Function copied from Docker CLI, since importing the Docker CLI broke our go modules.