dubboctl: consider extracting the template dockerfile contents into separate files under a dir

[yin1999]

The template dockerfile content is now placed in a go source file. Consider extracting the template dockerfile content into separate dockerfiles, like go.dockerfile, java.dockerfile, and using go:embed to embed dockerfile content as string variables in a index go source file.

By this way, we could get dockerfile highlighting feature provided by IDE and it will be easier to use tools to check the syntax and format the source file. It also makes it easier for us to support more runtimes, modify individual runtime configuration files, etc.