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