Request support for more source images via Config

[cdhunt]

My motivation is to include support for Windows containers. I think all that's required is making the following values configurable. At least for dotnet.

package/dotnet/Metaparticle.Package/Metaparticle.cs

Lines 135 to 136 in 89cc317

	instructions.Add(new Instruction("FROM", "debian:9"));
	instructions.Add(new Instruction("RUN", " apt-get update && apt-get -qq -y install libunwind8 libicu57 libssl1.0 liblttng-ust0 libcurl3 libuuid1 libkrb5-3 zlib1g"));

This was mentioned in #53 and #51. I don't think #51 addresses this sufficiently. If I have to maintain a separate Dockerfile, then I lose much of the value of Metaparticle.

[brendandburns]

Yeah, I can definitely see introducing the ability to customize the 'FROM' makes a ton of sense.

Want to do the PR?