Provide current env as variables in eval context

Signed-off-by: Patrick Van Stee <patrick@vanstee.me>

bake/hcl.go

@@ -1,10 +1,27 @@
 package bake
 
-import "github.com/hashicorp/hcl/v2/hclsimple"
+import (
+	"os"
+	"strings"
+
+	hcl "github.com/hashicorp/hcl/v2"
+	"github.com/hashicorp/hcl/v2/hclsimple"
+	"github.com/zclconf/go-cty/cty"
+)
 
 func ParseHCL(dt []byte, fn string) (*Config, error) {
+	variables := make(map[string]cty.Value)
+	for _, env := range os.Environ() {
+		parts := strings.SplitN(env, "=", 2)
+		variables[parts[0]] = cty.StringVal(parts[1])
+	}
+
+	ctx := &hcl.EvalContext{
+		Variables: variables,
+	}
+
 	var c Config
-	if err := hclsimple.Decode(fn, dt, nil, &c); err != nil {
+	if err := hclsimple.Decode(fn, dt, ctx, &c); err != nil {
 		return nil, err
 	}
 	return &c, nil

bake/hcl_test.go

@@ -1,12 +1,15 @@
 package bake
 
 import (
+	"os"
 	"testing"
 
 	"github.com/stretchr/testify/require"
 )
 
 func TestParseHCL(t *testing.T) {
+	os.Setenv("IAMCROSS", "true")
+
 	var dt = []byte(`
 	group "default" {
 		targets = ["db", "webapp"]
@@ -35,7 +38,7 @@ func TestParseHCL(t *testing.T) {
 	target "webapp-plus" {
 		inherits = ["webapp", "cross"]
 		args = {
-			IAMCROSS = "true"
+			IAMCROSS = "${IAMCROSS}"
 		}
 	}
 	`)
@@ -58,4 +61,8 @@ func TestParseHCL(t *testing.T) {
 	require.Equal(t, c.Targets[2].Name, "cross")
 	require.Equal(t, 2, len(c.Targets[2].Platforms))
 	require.Equal(t, []string{"linux/amd64", "linux/arm64"}, c.Targets[2].Platforms)
+
+	require.Equal(t, c.Targets[3].Name, "webapp-plus")
+	require.Equal(t, 1, len(c.Targets[3].Args))
+	require.Equal(t, map[string]string{"IAMCROSS": "true"}, c.Targets[3].Args)
 }

go.mod

@@ -55,6 +55,7 @@ require (
 	github.com/theupdateframework/notary v0.6.1 // indirect
 	github.com/tonistiigi/units v0.0.0-20180711220420-6950e57a87ea
 	github.com/xlab/handysort v0.0.0-20150421192137-fb3537ed64a1 // indirect
+	github.com/zclconf/go-cty v1.2.0
 	golang.org/x/sync v0.0.0-20190911185100-cd5d95a43a6e
 	gopkg.in/dancannon/gorethink.v3 v3.0.5 // indirect
 	gopkg.in/fatih/pool.v2 v2.0.0 // indirect