Add support for custom Dockerfile template

[bchassoul]

No description provided.

[sylane]

This is definitely a great addition, but looking at where you needed that for, wouldn't that be better to add configuration to better customise the default template ? Otherwise, instead of making the plugin more powerful and customisable, all the project will progressively use their own templates, then why wouldn't they use Dockerfile directly instead ?

Looking quickly at the custom templates you used It for, it seems the only extra config needed to be able to use the default template would be:

• Add way to extend the files copied into the builder (line 24) and maybe change the default template as it would make sense for it to include c_src, src, priv, config and optional _checkouts anyway.
• Add config to change the entry point, with the possibility to specify a custom script...

This PR is still very useful, but I am a bit worried that if a custom template is the easy way out of a missing feature, the plugin is kind of loosing its meaning.

[sylane]

space ?