feat: add docker recipe + environment with default interpolation in config

internal/config/environment.go

@@ -10,7 +10,41 @@ import (
 	"gopkg.in/yaml.v3"
 )
 
-var reVar = regexp.MustCompile(`^\${(\w+)}$`)
+var (
+	interpolationRegExp          = regexp.MustCompile(`^\${((?P<varName>\w+)|((?P<varNameWithDefault>\w+):-(?P<defaultValue>[^}]+)))}$`)
+	varNameGroupIndex            = interpolationRegExp.SubexpIndex("varName")
+	varNameWithDefaultGroupIndex = interpolationRegExp.SubexpIndex("varNameWithDefault")
+	defaultValueGroupIndex       = interpolationRegExp.SubexpIndex("defaultValue")
+)
+
+func interpolate(str string, getValueFunc func(name string) string) string {
+	for _, match := range interpolationRegExp.FindAllStringSubmatch(str, -1) {
+		varName := match[varNameWithDefaultGroupIndex]
+		if varName == "" {
+			varName = match[varNameGroupIndex]
+		}
+
+		if varName == "" {
+			continue
+		}
+
+		defaultValue := ""
+		if defaultValueGroupIndex < len(match) {
+			defaultValue = match[defaultValueGroupIndex]
+		}
+
+		str = getValueFunc(varName)
+		if str == "" {
+			str = defaultValue
+		}
+	}
+
+	return str
+}
+
+func interpolateEnv(str string) string {
+	return interpolate(str, os.Getenv)
+}
 
 type InterpolatedString string
 
@@ -21,11 +55,7 @@ func (is *InterpolatedString) UnmarshalYAML(value *yaml.Node) error {
 		return errors.WithStack(err)
 	}
 
-	if match := reVar.FindStringSubmatch(str); len(match) > 0 {
-		*is = InterpolatedString(os.Getenv(match[1]))
-	} else {
-		*is = InterpolatedString(str)
-	}
+	*is = InterpolatedString(interpolateEnv(str))
 
 	return nil
 }
@@ -39,9 +69,7 @@ func (ii *InterpolatedInt) UnmarshalYAML(value *yaml.Node) error {
 		return errors.Wrapf(err, "could not decode value '%v' (line '%d') into string", value.Value, value.Line)
 	}
 
-	if match := reVar.FindStringSubmatch(str); len(match) > 0 {
-		str = os.Getenv(match[1])
-	}
+	str = interpolateEnv(str)
 
 	intVal, err := strconv.ParseInt(str, 10, 32)
 	if err != nil {
@@ -62,9 +90,7 @@ func (ib *InterpolatedBool) UnmarshalYAML(value *yaml.Node) error {
 		return errors.Wrapf(err, "could not decode value '%v' (line '%d') into string", value.Value, value.Line)
 	}
 
-	if match := reVar.FindStringSubmatch(str); len(match) > 0 {
-		str = os.Getenv(match[1])
-	}
+	str = interpolateEnv(str)
 
 	boolVal, err := strconv.ParseBool(str)
 	if err != nil {
@@ -91,9 +117,7 @@ func (im *InterpolatedMap) UnmarshalYAML(value *yaml.Node) error {
 			continue
 		}
 
-		if match := reVar.FindStringSubmatch(strVal); len(match) > 0 {
-			strVal = os.Getenv(match[1])
-		}
+		strVal = interpolateEnv(strVal)
 
 		data[key] = strVal
 	}
@@ -113,9 +137,7 @@ func (iss *InterpolatedStringSlice) UnmarshalYAML(value *yaml.Node) error {
 	}
 
 	for index, value := range data {
-		if match := reVar.FindStringSubmatch(value); len(match) > 0 {
-			value = os.Getenv(match[1])
-		}
+		value = interpolateEnv(value)
 
 		data[index] = value
 	}
@@ -134,9 +156,7 @@ func (id *InterpolatedDuration) UnmarshalYAML(value *yaml.Node) error {
 		return errors.Wrapf(err, "could not decode value '%v' (line '%d') into string", value.Value, value.Line)
 	}
 
-	if match := reVar.FindStringSubmatch(str); len(match) > 0 {
-		str = os.Getenv(match[1])
-	}
+	str = interpolateEnv(str)
 
 	duration, err := time.ParseDuration(str)
 	if err != nil {

internal/config/environment_test.go

@@ -0,0 +1,63 @@
+package config
+
+import (
+	"fmt"
+	"testing"
+)
+
+type interpolateTestCase struct {
+	String   string
+	Data     map[string]string
+	Expected string
+}
+
+var interpolateTestCases = []interpolateTestCase{
+	{
+		String: "${foo}",
+		Data: map[string]string{
+			"foo": "bar",
+		},
+		Expected: "bar",
+	},
+	{
+		String:   "${hello:-world}",
+		Data:     map[string]string{},
+		Expected: "world",
+	},
+	{
+		String:   "${hello:-}",
+		Data:     map[string]string{},
+		Expected: "${hello:-}",
+	},
+	{
+		String:   "foo",
+		Data:     map[string]string{},
+		Expected: "foo",
+	},
+	{
+		String:   "",
+		Data:     map[string]string{},
+		Expected: "",
+	},
+}
+
+func TestInterpolate(t *testing.T) {
+	for idx, tc := range interpolateTestCases {
+		func(idx int, tc interpolateTestCase) {
+			t.Run(fmt.Sprintf("Case_%d", idx), func(t *testing.T) {
+				result := interpolate(tc.String, func(name string) string {
+					value, exists := tc.Data[name]
+					if !exists {
+						return ""
+					}
+
+					return value
+				})
+
+				if e, g := tc.Expected, result; e != g {
+					t.Errorf("result: expected '%v', got '%v'", tc.Expected, result)
+				}
+			})
+		}(idx, tc)
+	}
+}