Prevent Docker from running commands as root user

[XzAeRo]

Docker runs all commands as root user. This is a huge security issue, and should be avoided.

To fix it, we can modify the Dockerfiles to a format like this:

# run here all commands related to system wide installations, such as pecl packages.

# setup non-root user
RUN useradd --create-home appuser
WORKDIR /home/appuser
USER appuser

# run the rest of the commands as usual but without root access