feat: environment configuration

Environment variables can now be used to configure any
config property including the feeds list.

This makes it easier to use in environments without access
to config files like GCR. Some may prefer not to have a separate
config file as well.

Also added build metadata to the docker image and binaries.

internal/envextended/provider.go

@@ -0,0 +1,105 @@
+package envextended
+
+import (
+	"errors"
+	"os"
+	"strings"
+
+	"github.com/tidwall/sjson"
+)
+
+type Env struct {
+	prefix string
+	delim  string
+	cb     func(key string, value string) (string, interface{})
+	out    string
+}
+
+func Provider(prefix, delim string, cb func(s string) string) *Env {
+	e := &Env{
+		prefix: prefix,
+		delim:  delim,
+		out:    "{}",
+	}
+	if cb != nil {
+		e.cb = func(key string, value string) (string, interface{}) {
+			return cb(key), value
+		}
+	}
+	return e
+}
+
+// ProviderWithValue works exactly the same as Provider except the callback
+// takes a (key, value) with the variable name and value and allows you
+// to modify both. This is useful for cases where you may want to return
+// other types like a string slice instead of just a string.
+func ProviderWithValue(prefix, delim string, cb func(key string, value string) (string, interface{})) *Env {
+	return &Env{
+		prefix: prefix,
+		delim:  delim,
+		cb:     cb,
+	}
+}
+
+// ReadBytes reads the contents of a file on disk and returns the bytes.
+func (e *Env) ReadBytes() ([]byte, error) {
+	// Collect the environment variable keys.
+	var keys []string
+	for _, k := range os.Environ() {
+		if e.prefix != "" {
+			if strings.HasPrefix(k, e.prefix) {
+				keys = append(keys, k)
+			}
+		} else {
+			keys = append(keys, k)
+		}
+	}
+
+	for _, k := range keys {
+		parts := strings.SplitN(k, "=", 2)
+
+		var (
+			key   string
+			value interface{}
+		)
+
+		// If there's a transformation callback,
+		// run it through every key/value.
+		if e.cb != nil {
+			key, value = e.cb(parts[0], parts[1])
+			// If the callback blanked the key, it should be omitted
+			if key == "" {
+				continue
+			}
+		} else {
+			key = parts[0]
+			value = parts[1]
+		}
+
+		if err := e.set(key, value); err != nil {
+			return []byte{}, err
+		}
+	}
+
+	if e.out == "" {
+		return []byte("{}"), nil
+	}
+
+	return []byte(e.out), nil
+}
+
+func (e *Env) set(key string, value interface{}) error {
+	out, err := sjson.Set(e.out, strings.Replace(key, e.delim, ".", -1), value)
+	if err != nil {
+		return err
+	}
+
+	e.out = out
+
+	return nil
+}
+
+// Read is not supported by the file provider.
+func (e *Env) Read() (map[string]interface{}, error) {
+	return nil, errors.New("envextended provider does not support this method")
+}