[FEATURE REQ] Support custom build commands

[Clockwork-Muse]

Description.

For modern containerized application development, there's a few different ways to get completed container:

• Manually write and maintain one or more Dockerfiles.
• Use something like pack and the whole buildpack ecosystem to simplify the process (potentially not needing any configuration).
• Use the language's standard build tool to create the image. This may require a plugin for some systems/tools (gradle for Java), but may just be built-in now (C#). May use pack under the covers.

It would be great if, instead of needing to manually maintain our own images (and potentially multiple different images for each target!), it would be great if instead the system could use buildpacks, or at minimum support custom build commands that could be used to do so.