builder/remotecontext: use lowercase for error

Errors should not be capitalized. This error was marked as
"being compatible" with the old error, However, looking at
the original error that was in place before d1faf3df27207af111daf4bd0191478c374dbc55,
I noticed that the error was lowercase before, so it should
be ok to change it back to be lowercase.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

builder/dockerfile/internals_test.go

@@ -50,7 +50,7 @@ func TestDockerfileOutsideTheBuildContext(t *testing.T) {
 	contextDir, cleanup := createTestTempDir(t, "", "builder-dockerfile-test")
 	defer cleanup()
 
-	expectedError := "Forbidden path outside the build context: ../../Dockerfile ()"
+	expectedError := "path outside the build context: ../../Dockerfile ()"
 	if runtime.GOOS == "windows" {
 		expectedError = "failed to resolve scoped path ../../Dockerfile ()"
 	}

builder/remotecontext/detect.go

@@ -185,7 +185,7 @@ func FullPath(remote builder.Source, path string) (string, error) {
 		if runtime.GOOS == "windows" {
 			return "", fmt.Errorf("failed to resolve scoped path %s (%s): %s. Possible cause is a forbidden path outside the build context", path, fullPath, err)
 		}
-		return "", fmt.Errorf("Forbidden path outside the build context: %s (%s)", path, fullPath) // backwards compat with old error
+		return "", fmt.Errorf("forbidden path outside the build context: %s (%s)", path, fullPath) // backwards compat with old error
 	}
 	return fullPath, nil
 }