Fix "docker ps" with no containers regression

The header row was not being printed when "docker ps" was invoked without containers thanks to the new format support, and we instead received a single blank line.

Signed-off-by: Andrew "Tianon" Page <admwiggin@gmail.com>
Upstream-commit: f57fc03e3b39c225a05edfe217bd7616949d0dd0
Component: engine

components/engine/api/client/ps/custom.go

@@ -191,6 +191,13 @@ func customFormat(ctx Context, containers []types.Container) {
 	}
 
 	if table {
+		if len(header) == 0 {
+			// if we still don't have a header, we didn't have any containers so we need to fake it to get the right headers from the template
+			containerCtx := &containerContext{}
+			tmpl.Execute(bytes.NewBufferString(""), containerCtx)
+			header = containerCtx.fullHeader()
+		}
+
 		t := tabwriter.NewWriter(ctx.Output, 20, 1, 3, ' ', 0)
 		t.Write([]byte(header))
 		t.Write([]byte("\n"))