How to give folder permissions inside a docker container Folder

As the Other user already pointed out, move USER admin to a later step

FROM python:2.7
RUN pip install Flask==0.11.1 
RUN useradd -ms /bin/bash admin
COPY --chown=admin:admin app /app
WORKDIR /app
USER admin
CMD ["python", "app.py"] 

For versions release v17.09.0-ce and newer you can use the optional flag --chown=<user>:<group> with either the ADD or COPY commands.

For example

COPY --chown=<user>:<group> <hostPath> <containerPath>

The documentation for the --chown flag is documented on Dockerfile Reference page.

I guess you are switching to user "admin" which doesn't have the ownership to change permissions on /app directory. Change the ownership using "root" user. Below Dockerfile worked for me -

FROM python:2.7
RUN pip install Flask==0.11.1 
RUN useradd -ms /bin/bash admin
COPY app /app
WORKDIR /app
RUN chown -R admin:admin /app
RUN chmod 755 /app
USER admin
CMD ["python", "app.py"] 

PS - Try to get rid of "777" permission. I momentarily tried to do it in above Dockerfile.