Cannot execute RUN mkdir in a Dockerfile
Solution 1:
The problem is that /var/www doesn't exist either, and mkdir isn't recursive by default -- it expects the immediate parent directory to exist.
Use:
mkdir -p /var/www/app
...or install a package that creates a /var/www prior to reaching this point in your Dockerfile.
Solution 2:
When creating subdirectories hanging off from a non-existing parent directory(s) you must pass the -p flag to mkdir ... Please update your Dockerfile with
RUN mkdir -p ...
I tested this and it's correct.