Create demo/log/.keep

[hirowatari]

Ensure that log directory exists. Prevent docker build from failing.

I was getting the error:

 => ERROR [polaris stage-2 5/6] RUN useradd rails --create-home --shell /bin/  0.3s
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 > [polaris stage-2 5/6] RUN useradd rails --create-home --shell /bin/bash &&     chown -R rails:rails db log tmp:
0.248 chown: cannot access 'log': No such file or directory
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failed to solve: process "/bin/sh -c useradd rails --create-home --shell /bin/bash &&     chown -R rails:rails db log tmp" did not complete successfully: exit code: 1

Creating the directory fixed the issue and allowed me to docker build