Fix parsing invalid Dockerfile (#4943)

Author: gsquared94

pkg/skaffold/docker/dependencies_test.go

@@ -216,6 +216,11 @@ FROM stage
 ADD ./file /etc/file
 `
 
+const invalidFrom = `
+FROM
+COPY . /
+`
+
 type fakeImageFetcher struct{}
 
 func (f *fakeImageFetcher) fetch(image string, _ Config) (*v1.ConfigFile, error) {
@@ -530,6 +535,12 @@ func TestGetDependencies(t *testing.T) {
 			ignoreFilename: "Dockerfile.dockerignore",
 			expected:       []string{".dot", "Dockerfile", "Dockerfile.dockerignore", "file", "server.go", "test.conf", "worker.go"},
 		},
+		{
+			description: "invalid dockerfile",
+			dockerfile:  invalidFrom,
+			workspace:   ".",
+			shouldErr:   true,
+		},
 	}
 
 	for _, test := range tests {

pkg/skaffold/docker/parse.go

@@ -27,6 +27,7 @@ import (
 
 	v1 "github.com/google/go-containerregistry/pkg/v1"
 	"github.com/moby/buildkit/frontend/dockerfile/command"
+	"github.com/moby/buildkit/frontend/dockerfile/instructions"
 	"github.com/moby/buildkit/frontend/dockerfile/parser"
 	"github.com/moby/buildkit/frontend/dockerfile/shell"
 	"github.com/sirupsen/logrus"
@@ -75,6 +76,11 @@ func readCopyCmdsFromDockerfile(onlyLastImage bool, absDockerfilePath, workspace
 		return nil, fmt.Errorf("parsing dockerfile %q: %w", absDockerfilePath, err)
 	}
 
+	// instructions.Parse will check for malformed Dockerfile
+	if _, _, err := instructions.Parse(res.AST); err != nil {
+		return nil, fmt.Errorf("parsing dockerfile %q: %w", absDockerfilePath, err)
+	}
+
 	dockerfileLines := res.AST.Children
 
 	if err := expandBuildArgs(dockerfileLines, buildArgs); err != nil {