Allow creating FileBuilder without specifying initial command

[macsux]

Currently FileBuilder is started from ImageBuilder via From method, but this adds first "command" to the sub-builder. This gets awkward when adding conditional buildup of container image, where first command to be added is not easily identified. This requires tricks like the use of a local function:

FileBuilder From(string image) => fileBuilder == null
            ? builder.From(image)
            : fileBuilder.From(image);

It would be better if FileBuilder can be started without adding the initial command.

[mariotoffia]

I've compiled a pre-release - please have a go https://www.nuget.org/packages/Ductus.FluentDocker/2.8.8-beta.14

[mariotoffia]

@macsux Please report back if it works or if you have any issues.

[macsux]

I think relying on From with empty string to implement this is awkward. I suggest adding a new method like DockerfileBuilder to ImageBuilder.

[mariotoffia]

I would say the this is consistent how the API do work e.g. public ImageBuilder DefineImage(string image = null). I do not say it is optimal and many things will change when 2.0.0 is released but for now I try to stick to be as consistent as possible.