Add support for comment in .dockerignore

This fix tries to address the issue raised in #20083 where
comment is not supported in `.dockerignore`.

This fix updated the processing of `.dockerignore` so that any
lines starting with `#` are ignored, which is similiar to the
behavior of `.gitignore`.

Related documentation has been updated.

Additional tests have been added to cover the changes.

This fix fixes #20083.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

integration-cli/docker_cli_build_test.go

@@ -6755,3 +6755,39 @@ func (s *DockerSuite) TestBuildDeleteCommittedFile(c *check.C) {
 		c.Fatal(err)
 	}
 }
+
+// #20083
+func (s *DockerSuite) TestBuildDockerignoreComment(c *check.C) {
+	name := "testbuilddockerignorecleanpaths"
+	dockerfile := `
+        FROM busybox
+        ADD . /tmp/
+        RUN sh -c "(ls -la /tmp/#1)"
+        RUN sh -c "(! ls -la /tmp/#2)"
+        RUN sh -c "(! ls /tmp/foo) && (! ls /tmp/foo2) && (ls /tmp/dir1/foo)"`
+	ctx, err := fakeContext(dockerfile, map[string]string{
+		"foo":      "foo",
+		"foo2":     "foo2",
+		"dir1/foo": "foo in dir1",
+		"#1":       "# file 1",
+		"#2":       "# file 2",
+		".dockerignore": `# Visual C++ cache files
+# because we have git ;-)
+# The above comment is from #20083
+foo
+#dir1/foo
+foo2
+# The following is considered as comment as # is at the beginning
+#1
+# The following is not considered as comment as # is not at the beginning
+  #2
+`,
+	})
+	if err != nil {
+		c.Fatal(err)
+	}
+	defer ctx.Close()
+	if _, err := buildImageFromContext(name, ctx, true); err != nil {
+		c.Fatal(err)
+	}
+}