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

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. For example,

```console
faas build --builder native
```

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.

buildpacks/builder.go

@@ -30,10 +30,17 @@ var Runtimes = map[string]string{
 
 // Build the Function at path.
 func (builder *Builder) Build(f faas.Function) (err error) {
-	// dervive the builder from the specificed runtime
-	packBuilder, ok := Runtimes[f.Runtime]
-	if !ok {
-		return errors.New(fmt.Sprint("unsupported runtime: ", f.Runtime))
+
+	// Use the builder found in the Function configuration file
+	// If one isn't found, use the defaults
+	var packBuilder string
+	if f.Builder != "" {
+		packBuilder = f.Builder
+	} else {
+		packBuilder = Runtimes[f.Runtime]
+		if packBuilder == "" {
+			return errors.New(fmt.Sprint("unsupported runtime: ", f.Runtime))
+		}
 	}
 
 	// Build options for the pack client.

client.go

@@ -4,7 +4,11 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"os"
+	"path/filepath"
+
+	"gopkg.in/yaml.v2"
 )
 
 const (
@@ -348,6 +352,16 @@ func (c *Client) Initialize(cfg Function) (err error) {
 		return
 	}
 
+	// Check if template specifies a builder image. If so, add to configuration
+	if builderConfig, err := ioutil.ReadFile(filepath.Join(f.Root, ".builder.yaml")); err == nil {
+		// A .builder file was found. Read the default builder and set in the config file
+		// TODO: A command line flag could be used to specify non-default builders
+		builders := make(map[string]string)
+		if err := yaml.Unmarshal(builderConfig, builders); err == nil {
+			f.Builder = builders["default"]
+		}
+	}
+
 	// Write out the config.
 	if err = writeConfig(f); err != nil {
 		return

config.go

@@ -19,6 +19,7 @@ type config struct {
 	Runtime   string `yaml:"runtime"`
 	Image     string `yaml:"image"`
 	Trigger   string `yaml:"trigger"`
+	Builder   string `yaml:"builder"`
 	// Add new values to the toConfig/fromConfig functions.
 }
 
@@ -53,6 +54,7 @@ func fromConfig(c config) (f Function) {
 		Runtime:   c.Runtime,
 		Image:     c.Image,
 		Trigger:   c.Trigger,
+		Builder:   c.Builder,
 	}
 }
 
@@ -64,6 +66,7 @@ func toConfig(f Function) config {
 		Runtime:   f.Runtime,
 		Image:     f.Image,
 		Trigger:   f.Trigger,
+		Builder:   f.Builder,
 	}
 }
 

function.go

@@ -42,6 +42,9 @@ type Function struct {
 	// If Image is provided, it overrides the default of concatenating
 	// "Repo+Name:latest" to derive the Image.
 	Image string
+
+	// Builder represents the CNCF Buildpack builder image for a function
+	Builder string
 }
 
 // NewFunction loads a Function from a path on disk. use .Initialized() to determine if

templates/go/events/.builder.yaml

@@ -0,0 +1 @@
+default: nodejs quay.io/boson/faas-go-builder

templates/go/http/.builder.yaml

@@ -0,0 +1 @@
+default: quay.io/boson/faas-go-builder

templates/node/events/.builder.yaml

@@ -0,0 +1 @@
+default: quay.io/boson/faas-nodejs-builder

templates/node/http/.builder.yaml

@@ -0,0 +1 @@
+default: quay.io/boson/faas-nodejs-builder

templates/quarkus/events/.builder.yaml

@@ -0,0 +1 @@
+default: quay.io/boson/faas-quarkus-builder

templates/quarkus/http/.builder.yaml

@@ -0,0 +1 @@
+default: quay.io/boson/faas-quarkus-builder