Handle case of missing GH variables (#492)

Closes #482 

I traced the problem to where buildx parses the cli args
([here](fa4461b9a1/util/buildflags/cache.go (L14))),
and confirmed it applies defaults based on GHA environment variables and
ignores the cacheTo/cacheFrom directive altogether when the variables
aren't available.

The fix is to ignore the GHA cache directive when it the upstream parser
ignores it.

provider/internal/image_test.go

@@ -25,6 +25,7 @@ import (
 	_ "github.com/docker/buildx/driver/docker-container"
 
 	"github.com/distribution/reference"
+	pb "github.com/docker/buildx/controller/pb"
 	"github.com/moby/buildkit/client"
 	"github.com/moby/buildkit/exporter/containerimage/exptypes"
 	"github.com/regclient/regclient/types/descriptor"
@@ -818,7 +819,6 @@ func TestImageDiff(t *testing.T) {
 }
 
 func TestValidateImageArgs(t *testing.T) {
-	t.Parallel()
 	t.Run("invalid inputs", func(t *testing.T) {
 		t.Parallel()
 		args := ImageArgs{
@@ -917,6 +917,85 @@ func TestValidateImageArgs(t *testing.T) {
 		assert.Len(t, opts.Exports, 0)
 	})
 
+	t.Run("environment variables", func(t *testing.T) {
+		tests := []struct {
+			name          string
+			envs          map[string]string
+			args          ImageArgs
+			wantCacheFrom *pb.CacheOptionsEntry
+			wantCacheTo   *pb.CacheOptionsEntry
+		}{
+			{
+				name: "gha environment",
+				envs: map[string]string{
+					"ACTIONS_CACHE_URL":     "test-cache-url",
+					"ACTIONS_RUNTIME_TOKEN": "test-runtime-token",
+				},
+				args: ImageArgs{
+					Context:   &BuildContext{Context: Context{Location: "testdata/noop"}},
+					CacheFrom: []CacheFrom{{GHA: &CacheFromGitHubActions{}}},
+					CacheTo: []CacheTo{{GHA: &CacheToGitHubActions{
+						CacheFromGitHubActions: CacheFromGitHubActions{},
+					}}},
+				},
+				wantCacheFrom: &pb.CacheOptionsEntry{
+					Type: "gha",
+					Attrs: map[string]string{
+						"token": "test-runtime-token",
+						"url":   "test-cache-url",
+					},
+				},
+				wantCacheTo: &pb.CacheOptionsEntry{
+					Type: "gha",
+					Attrs: map[string]string{
+						"token": "test-runtime-token",
+						"url":   "test-cache-url",
+					},
+				},
+			},
+			{
+				name: "non-gha environment",
+				envs: map[string]string{
+					"ACTIONS_CACHE_URL":     "",
+					"ACTIONS_RUNTIME_TOKEN": "",
+				},
+				args: ImageArgs{
+					Context:   &BuildContext{Context: Context{Location: "testdata/noop"}},
+					CacheFrom: []CacheFrom{{GHA: &CacheFromGitHubActions{}}},
+					CacheTo: []CacheTo{{GHA: &CacheToGitHubActions{
+						CacheFromGitHubActions: CacheFromGitHubActions{},
+					}}},
+				},
+				wantCacheFrom: nil,
+				wantCacheTo:   nil,
+			},
+		}
+
+		for _, tt := range tests {
+			t.Run(tt.name, func(t *testing.T) {
+				for k, v := range tt.envs {
+					t.Setenv(k, v)
+				}
+				validate := func(preview bool) {
+					opts, err := tt.args.validate(true, preview)
+					require.NoError(t, err)
+					if tt.wantCacheFrom != nil {
+						assert.Equal(t, tt.wantCacheFrom, opts.CacheFrom[0])
+					} else {
+						assert.Len(t, opts.CacheFrom, 0)
+					}
+					if tt.wantCacheTo != nil {
+						assert.Equal(t, tt.wantCacheTo, opts.CacheTo[0])
+					} else {
+						assert.Len(t, opts.CacheTo, 0)
+					}
+				}
+				validate(true)
+				validate(false)
+			})
+		}
+	})
+
 	t.Run("multiple exports pre-0.13", func(t *testing.T) {
 		t.Parallel()
 		args := ImageArgs{