Dockerfile cannot find executable script (no such file or directory) - docker

Question

I'm writting a Dockerfile in order to create an image for a web server (a shiny server more precisely). It works well, but it depends on a huge database folder (db/) that it is not distributed with the package, so I want to do all this preprocessing while creating the image, by running the corresponding script in the Dockerfile.
I expected this to be simple, but I'm struggling figuring out where my files are being located within the image.
This repo has the following structure:
Dockerfile
preprocessing_files
configuration_files
app/
application_files
db/
processed_files
So that app/db/ does not exist, but is created and filled with files when preprocessing_files are run.
The Dockerfile is the following:
# Install R version 3.6
FROM r-base:3.6.0
# Install Ubuntu packages
RUN apt-get update && apt-get install -y \
sudo \
gdebi-core \
pandoc \
pandoc-citeproc \
libcurl4-gnutls-dev \
libcairo2-dev/unstable \
libxml2-dev \
libxt-dev \
libssl-dev
# Download and install ShinyServer (latest version)
RUN wget --no-verbose https://s3.amazonaws.com/rstudio-shiny-server-os-build/ubuntu-12.04/x86_64/VERSION -O "version.txt" && \
VERSION=$(cat version.txt) && \
wget --no-verbose "https://s3.amazonaws.com/rstudio-shiny-server-os-build/ubuntu-12.04/x86_64/shiny-server-$VERSION-amd64.deb" -O ss-latest.deb && \
gdebi -n ss-latest.deb && \
rm -f version.txt ss-latest.deb
# Install R packages that are required
RUN R -e "install.packages(c('shiny', 'flexdashboard','rmarkdown','tidyverse','plotly','DT','drc','gridExtra','fitdistrplus'), repos='http://cran.rstudio.com/')"
# Copy configuration files into the Docker image
COPY shiny-server.conf /etc/shiny-server/shiny-server.conf
COPY /app /srv/shiny-server/
COPY /app/db /srv/shiny-server/app/
# Make the ShinyApp available at port 80
EXPOSE 80
CMD ["/usr/bin/shiny-server"]
This above file works well if preprocessing_files are run in advance, so app/application_files can successfully read app/db/processed_files. How could this script be run in the Dockerfile? To me the intuitive solution would be simply to write:
RUN bash -c "preprocessing.sh"
Before the ADD instruction, but then preprocessing_files are not found. If the above instruction is written below ADD and also WORKDIR app/, the same error happens. I cannot understand why.

Answer 1

You cannot execute code on the host machine from Dockerfile. RUN command executes inside the container being built. You can:
Copy preprocessing_files inside docker container and run preprocessing.sh inside the container (this would increase size of the container)
Create a makefile/build.sh script which launches preprocessing.sh before executing docker build