loader: restore implicit default of . for build context

In the Compose spec, as well as Compose v1.x, the `build.context`
field is mandatory in the object form of a `service.build` definition.

However, in compose-go (and thus Compose v2.x), this has never been
enforced, and an empty `build.context` field was implicitly set to
`.` aka the project directory.

Restore that behavior pending a decision on whether we want to make
the spec less restrictive here so that this is acceptable or change
`compose-go` to begin emitting a warning and eventually reject this.

Note that the order of normalization and resolving paths has been
switched so that normalization occurs first, and then paths can be
resolved across all fields consistently. This resulted in a small
test update where this was incorrect before - it was loading with
path resolution enabled but then asserting it had a relative path
for the env file.

Signed-off-by: Milas Bowman <milas.bowman@docker.com>

loader/loader.go

@@ -261,6 +261,13 @@ func Load(configDetails types.ConfigDetails, options ...func(*Options)) (*types.
 		Extensions:  model.Extensions,
 	}
 
+	if !opts.SkipNormalization {
+		err = Normalize(project)
+		if err != nil {
+			return nil, err
+		}
+	}
+
 	if opts.ResolvePaths {
 		err = ResolveRelativePaths(project)
 		if err != nil {
@@ -277,13 +284,6 @@ func Load(configDetails types.ConfigDetails, options ...func(*Options)) (*types.
 		}
 	}
 
-	if !opts.SkipNormalization {
-		err = Normalize(project)
-		if err != nil {
-			return nil, err
-		}
-	}
-
 	if !opts.SkipConsistencyCheck {
 		err = checkConsistency(project)
 		if err != nil {

loader/loader_test.go

@@ -1928,7 +1928,8 @@ func TestLoadWithExtends(t *testing.T) {
 
 	extendsDir := filepath.Join("testdata", "subdir")
 
-	expectedEnvFilePath := filepath.Join(extendsDir, "extra.env")
+	expectedEnvFilePath, err := filepath.Abs(filepath.Join(extendsDir, "extra.env"))
+	assert.NilError(t, err)
 
 	expServices := types.Services{
 		{

loader/normalize.go

@@ -29,6 +29,7 @@ import (
 
 // Normalize compose project by moving deprecated attributes to their canonical position and injecting implicit defaults
 func Normalize(project *types.Project) error {
+	// TODO(milas): this really belongs in ResolveRelativePaths
 	absWorkingDir, err := filepath.Abs(project.WorkingDir)
 	if err != nil {
 		return err
@@ -44,8 +45,7 @@ func Normalize(project *types.Project) error {
 		project.Networks["default"] = types.NetworkConfig{}
 	}
 
-	err = relocateExternalName(project)
-	if err != nil {
+	if err := relocateExternalName(project); err != nil {
 		return err
 	}
 
@@ -65,6 +65,9 @@ func Normalize(project *types.Project) error {
 		}
 
 		if s.Build != nil {
+			if s.Build.Context == "" {
+				s.Build.Context = "."
+			}
 			if s.Build.Dockerfile == "" && s.Build.DockerfileInline == "" {
 				s.Build.Dockerfile = "Dockerfile"
 			}

loader/normalize_test.go

@@ -218,3 +218,17 @@ func TestNormalizeImplicitDependencies(t *testing.T) {
 	assert.NilError(t, err)
 	assert.DeepEqual(t, expected, project.Services[0].DependsOn)
 }
+
+func TestImplicitContextPath(t *testing.T) {
+	project := &types.Project{
+		Name: "myProject",
+		Services: types.Services{
+			types.ServiceConfig{
+				Name:  "test",
+				Build: &types.BuildConfig{},
+			},
+		},
+	}
+	assert.NilError(t, Normalize(project))
+	assert.Equal(t, ".", project.Services[0].Build.Context)
+}