feat: set builder images in templates and .faas.yaml

[lance]

This commit adds a .builder.yaml file to each template directory. In the file
there is at the moment a single key/value pair, "default: <image>", where the
actual builder image name is <image>. Using a mapping allows the future
possibility that a user may specify a builder image by name via a flag on the
command line.

When a project is initialized, the .builder.yaml file is read, and the default
builder is saved in the project's .faas.yaml file. The .faas.yaml file is then
consulted when building an image with faas build. If the builder image is
specified, then the builder will use it. Otherwise, it will fallback to the
defaults. This allows developers to create custom builders, and specify them
in the configuration file.

An open question is whether the .builder.yaml file should be deleted after it
is read during project initialization. The init command is the only time the
file is consulted, and it may be confusing for the user if they discover a
second configuration-y file in the project directory.

Fixes: #120

[matejvasek]

An open question is whether the .builder.yaml file should be deleted after it
is read during project initialization. The init command is the only time the
file is consulted, and it may be confusing for the user if they discover a
second configuration-y file in the project directory.

I think it's OK to delete the file for now.

In future I will add a feature for build modes (like release, debug), that means the .builder.yaml should be renamed to.builders.yaml since the file will contain multiple builders. Question is whether this map of builder will be copied to .faas.yaml if so then it can be deleted, otherwise it would need to be preserved. But that is question for future, I'll take care that.

[matejvasek]

nodejs ?

[zroubalik]

LGTM, there's just the typo in the go events builder

+1 to delete the file afterwards

[matejvasek]

Runtimes var name seems odd in this context, maybe it could be Buildpacks or Runtime2Buildpack, I know you haven't introduced that name.

[zroubalik]

In future I will add a feature for build modes (like release, debug), that means the .builder.yaml should be renamed to.builders.yaml since the file will contain multiple builders. Question is whether this map of builder will be copied to .faas.yaml if so then it can be deleted, otherwise it would need to be preserved. But that is question for future, I'll take care that.

so what if we name the file .builders.yaml from the start, since we know that we will add more of them

[lance]

I've incorporated all of the feedback @zroubalik @matejvasek - ty!