bake: add --var flag for setting variable values

This provides alternative way to set values for Bake
variables without adding them the global environment variables.
This can also be used then environment variable access
is disabled with BUILDX_BAKE_DISABLE_VARS_ENV_LOOKUP.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

bake/compose.go

@@ -3,6 +3,7 @@ package bake
 import (
 	"context"
 	"fmt"
+	"maps"
 	"os"
 	"path/filepath"
 	"slices"
@@ -20,8 +21,8 @@ import (
 	"go.yaml.in/yaml/v3"
 )
 
-func ParseComposeFiles(fs []File) (*Config, error) {
-	envs, err := composeEnv()
+func ParseComposeFiles(fs []File, envOverrides map[string]string) (*Config, error) {
+	envs, err := composeEnv(envOverrides)
 	if err != nil {
 		return nil, err
 	}
@@ -278,8 +279,8 @@ func loadComposeFiles(cfgs []composetypes.ConfigFile, envs map[string]string, op
 	})
 }
 
-func validateComposeFile(dt []byte, fn string) (bool, error) {
-	envs, err := composeEnv()
+func validateComposeFile(dt []byte, fn string, envOverrides map[string]string) (bool, error) {
+	envs, err := composeEnv(envOverrides)
 	if err != nil {
 		return false, err
 	}
@@ -303,7 +304,7 @@ func validateCompose(dt []byte, envs map[string]string) error {
 	return err
 }
 
-func composeEnv() (map[string]string, error) {
+func composeEnv(envOverrides map[string]string) (map[string]string, error) {
 	var env []string
 	if envLookupAllowed() {
 		env = os.Environ()
@@ -315,6 +316,7 @@ func composeEnv() (map[string]string, error) {
 			return nil, err
 		}
 	}
+	maps.Copy(envs, envOverrides)
 	return envs, nil
 }