testplan: Make executable in Dockerfile.

- Needed to run Docker images in Recipes, similar to issue here:
https://stackoverflow.com/questions/59767901/standard-init-linux-go211-exec-user-process-caused-permission-denied.

BUG=b:182898188
TEST=CQ

Change-Id: I29a16627d2fbaa3df95cda021fb88577625f87fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/3116665
Commit-Queue: Andrew Lamb <andrewlamb@chromium.org>
Commit-Queue: Jaques Clapauch <jaquesc@google.com>
Tested-by: Andrew Lamb <andrewlamb@chromium.org>
Auto-Submit: Andrew Lamb <andrewlamb@chromium.org>
Reviewed-by: Jaques Clapauch <jaquesc@google.com>
diff --git a/src/chromiumos/test/plan/docker/Dockerfile b/src/chromiumos/test/plan/docker/Dockerfile
index da048e9..4bc2886 100644
--- a/src/chromiumos/test/plan/docker/Dockerfile
+++ b/src/chromiumos/test/plan/docker/Dockerfile
@@ -4,4 +4,5 @@
 
 FROM ubuntu:bionic
 COPY testplan /usr/bin/testplan
+RUN chmod +x /usr/bin/testplan
 ENTRYPOINT [ "/usr/bin/testplan" ]