[gen-dockerfile] Make /code directory creation idempotent

[tclasen]

Summary

Resolves:

• Default initial Dockerfile unable to build #1953
• Generated Dockerfile fails build because of mkdir /code #1930

How was it tested?

• Note: I don't recommend applying the patch below used to test this PR. Building an actual Dockerfile is way more of an integration test and I feel it is out of scope.

diff --git a/testscripts/generate/dockerfile.test.txt b/testscripts/generate/dockerfile.test.txt
index d63680b3..ec561564 100644
--- a/testscripts/generate/dockerfile.test.txt
+++ b/testscripts/generate/dockerfile.test.txt
@@ -1,3 +1,5 @@
 exec devbox init
 exec devbox generate dockerfile
 exists Dockerfile
+exec devbox add docker
+exec devbox run -- docker build .

Followed by:

$ go test ./testscripts -run TestScripts/dockerfile

which results in a successfully built dockerfile.

[mohsenari]

Thanks for this.