Add ability to push images without doing a build

[kfrance]

• balena CLI version: 10.25.16
• Operating system version: Arch Linux
• 32/64 bit OS and processor: 64-bit Intel processor
• Install method: npm
• If npm install, Node.js and npm version: e.g. Node v12.16.1 and npm v6.14.4

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I have projects where we are pushing to devices that are the same architecture as our development machines and we would like the ability to push to those devices without having to do a build. In the same way that I can deploy just an image to balena cloud without doing a build.

Currently to get around this, I have fake builds that add a random file to the build so that push has something to build. It just causes me to have extra dockerfiles for those fake builds.

[pdcastro]

• Thank you for this feature suggestion and for sharing the use case 👍
• Related to #1468 Support transferring pre-built images when balena push is used in local mode