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>

builder/dockerignore/dockerignore.go

@@ -20,7 +20,12 @@ func ReadAll(reader io.ReadCloser) ([]string, error) {
 	var excludes []string
 
 	for scanner.Scan() {
-		pattern := strings.TrimSpace(scanner.Text())
+		// Lines starting with # (comments) are ignored before processing
+		pattern := scanner.Text()
+		if strings.HasPrefix(pattern, "#") {
+			continue
+		}
+		pattern = strings.TrimSpace(pattern)
 		if pattern == "" {
 			continue
 		}

docs/reference/builder.md

@@ -379,9 +379,13 @@ the working and the root directory.  For example, the patterns
 in the `foo` subdirectory of `PATH` or in the root of the git
 repository located at `URL`.  Neither excludes anything else.
 
+If a line in `.dockerignore` file starts with `#` in column 1, then this line is
+considered as a comment and is ignored before interpreted by the CLI.
+
 Here is an example `.dockerignore` file:
 
 ```
+# comment
     */temp*
     */*/temp*
     temp?
@@ -391,6 +395,7 @@ This file causes the following build behavior:
 
 | Rule           | Behavior                                                                                                                                                                     |
 |----------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| `# comment`    | Ignored.                 |
 | `*/temp*`      | Exclude files and directories whose names start with `temp` in any immediate subdirectory of the root.  For example, the plain file `/somedir/temporary.txt` is excluded, as is the directory `/somedir/temp`.                 |
 | `*/*/temp*`    | Exclude files and directories starting with `temp` from any subdirectory that is two levels below the root. For example, `/somedir/subdir/temporary.txt` is excluded. |
 | `temp?`        | Exclude files and directories in the root directory whose names are a one-character extension of `temp`.  For example, `/tempa` and `/tempb` are excluded.

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)
+	}
+}