bake: initial set of composable bake attributes

This allows using either the csv syntax or object syntax to specify
certain attributes.

This applies to the following fields:
- output
- cache-from
- cache-to
- secret
- ssh

There are still some remaining fields to translate. Specifically
ulimits, annotations, and attest.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

bake/hcl_test.go

@@ -17,6 +17,7 @@ func TestHCLBasic(t *testing.T) {
 		target "db" {
 			context = "./db"
 			tags = ["docker.io/tonistiigi/db"]
+			output = ["type=image"]
 		}
 
 		target "webapp" {
@@ -25,6 +26,9 @@ func TestHCLBasic(t *testing.T) {
 			args = {
 				buildno = "123"
 			}
+			output = [
+				{ type = "image" }
+			]
 		}
 
 		target "cross" {
@@ -597,6 +601,113 @@ func TestHCLAttrsCustomType(t *testing.T) {
 	require.Equal(t, ptrstr("linux/arm64"), c.Targets[0].Args["v1"])
 }
 
+func TestHCLAttrsCapsuleType(t *testing.T) {
+	dt := []byte(`
+	target "app" {
+		cache-from = [
+			{ type = "registry", ref = "user/app:cache" },
+			{ type = "local", src = "path/to/cache" },
+		]
+
+		cache-to = [
+			{ type = "local", dest = "path/to/cache" },
+		]
+
+		output = [
+			{ type = "oci", dest = "../out.tar" },
+		]
+
+		secret = [
+			{ id = "mysecret", src = "/local/secret" },
+			{ id = "mysecret2", env = "TOKEN" },
+		]
+
+		ssh = [
+			{ id = "default" },
+			{ id = "key", paths = ["path/to/key"] },
+		]
+	}
+	`)
+
+	c, err := ParseFile(dt, "docker-bake.hcl")
+	require.NoError(t, err)
+
+	require.Equal(t, 1, len(c.Targets))
+	require.Equal(t, []string{"type=oci,dest=../out.tar"}, stringify(c.Targets[0].Outputs))
+	require.Equal(t, []string{"type=local,src=path/to/cache", "user/app:cache"}, stringify(c.Targets[0].CacheFrom))
+	require.Equal(t, []string{"type=local,dest=path/to/cache"}, stringify(c.Targets[0].CacheTo))
+	require.Equal(t, []string{"id=mysecret,src=/local/secret", "id=mysecret2,env=TOKEN"}, stringify(c.Targets[0].Secrets))
+	require.Equal(t, []string{"default", "key=path/to/key"}, stringify(c.Targets[0].SSH))
+}
+
+func TestHCLAttrsCapsuleTypeVars(t *testing.T) {
+	dt := []byte(`
+	variable "foo" {
+		default = "bar"
+	}
+
+	target "app" {
+		cache-from = [
+			{ type = "registry", ref = "user/app:cache" },
+			{ type = "local", src = "path/to/cache" },
+		]
+
+		cache-to = [ target.app.cache-from[0] ]
+
+		output = [
+			{ type = "oci", dest = "../out.tar" },
+		]
+
+		secret = [
+			{ id = "mysecret", src = "/local/secret" },
+		]
+
+		ssh = [
+			{ id = "default" },
+			{ id = "key", paths = ["path/to/${target.app.output[0].type}"] },
+		]
+	}
+
+	target "web" {
+		cache-from = target.app.cache-from
+
+		output = [ "type=oci,dest=../${foo}.tar" ]
+
+		secret = [
+			{ id = target.app.output[0].type, src = "/local/secret" },
+		]
+	}
+	`)
+
+	c, err := ParseFile(dt, "docker-bake.hcl")
+	require.NoError(t, err)
+
+	require.Equal(t, 2, len(c.Targets))
+
+	findTarget := func(t *testing.T, name string) *Target {
+		t.Helper()
+		for _, tgt := range c.Targets {
+			if tgt.Name == name {
+				return tgt
+			}
+		}
+		t.Fatalf("could not find target %q", name)
+		return nil
+	}
+
+	app := findTarget(t, "app")
+	require.Equal(t, []string{"type=oci,dest=../out.tar"}, stringify(app.Outputs))
+	require.Equal(t, []string{"type=local,src=path/to/cache", "user/app:cache"}, stringify(app.CacheFrom))
+	require.Equal(t, []string{"user/app:cache"}, stringify(app.CacheTo))
+	require.Equal(t, []string{"id=mysecret,src=/local/secret"}, stringify(app.Secrets))
+	require.Equal(t, []string{"default", "key=path/to/oci"}, stringify(app.SSH))
+
+	web := findTarget(t, "web")
+	require.Equal(t, []string{"type=oci,dest=../bar.tar"}, stringify(web.Outputs))
+	require.Equal(t, []string{"type=local,src=path/to/cache", "user/app:cache"}, stringify(web.CacheFrom))
+	require.Equal(t, []string{"id=oci,src=/local/secret"}, stringify(web.Secrets))
+}
+
 func TestHCLMultiFileAttrs(t *testing.T) {
 	dt := []byte(`
 		variable "FOO" {