Add option for docker executor to handle stderr from results

Instead of always joining stdout and stderr, only join the two if the
caller explicitly asks for it. Otherwise, only return stdout.

plugins/providers/docker/driver.rb

@@ -20,9 +20,10 @@ module VagrantPlugins
       #
       # @return [String] id - ID matched from the docker build output.
       def build(dir, **opts, &block)
-        args   = Array(opts[:extra_args])
-        args   << dir
-        result = execute('docker', 'build', *args, &block)
+        args = Array(opts[:extra_args])
+        args << dir
+        opts = {with_stderr: true}
+        result = execute('docker', 'build', *args, opts, &block)
         matches = result.match(/Successfully built (?<id>.+)$/i)
         if !matches
           # Check for the new output format 'writing image sha256...'

plugins/providers/docker/executor/local.rb

@@ -27,10 +27,12 @@ module VagrantPlugins
               stdout: result.stdout
           end
 
-          if result.stdout.to_s.strip.length == 0
-            result.stderr
-          else
-            result.stdout
+          if opts
+            if opts[:with_stderr]
+              return result.stdout + " " + result.stderr
+            else
+              return result.stdout
+            end
           end
         end