Dockerfile with ONTEST instruction

Dockerfile

FROM "<base image>":"<version>"

WORKDIR "<path>"

# install packages required to run app
RUN apt-get update && apt-get install -y \
    "<app runtime> and <dependencies>" \  # add app runtime and required packages
    && rm -rf /var/lib/apt/lists/*

# install packages required to run tests   
ONTEST RUN apt-get update && apt-get install -y \
           "<test tools> and <dependencies>"    \     # add testing tools and required packages
           && rm -rf /var/lib/apt/lists/*

# copy app files
COPY app app
COPY run.sh run.sh

# copy test scripts
ONTEST COPY tests tests

# copy "main" test command
ONTEST COPY test.sh test.sh

# auto-generated volume for test results
# ONTEST VOLUME "/var/tests/results"

# ... EXPOSE, RUN, ADD ... for app and test environment

# main app command
CMD [run.sh, "<app arguments>"]

# main test command
ONTEST CMD [/test.sh, "<test arguments>"]