Define a build items for arbitrary instruction addition during dockerfile generation.

[iocanel]

Description

In the context Regenerating project files we would like to be able to regenerate docker files from the CLI.

In some cases it could be useful for extensions to contributed to additional instructions added during dockerfile generation.

Implementation ideas

So, we could have a MultiBuildItem that accepts an List of instructions that will be added to the generated dockerfile. The build item should also have an field (possibly enum) to select the target (jvm, native, native-micro).

Those build items could live in an SPI module of a new dockerfiles extension (similar to how kubernetes spi works).

[quarkus-bot[bot]]

/cc @geoand (kubernetes)

[iocanel]

/cc @cescoffier (regenerate project files)