shiny app docker container failed to launch in browser - r

Question

I'm trying to run my shiny app in a docker container.
My app folder structure is like this:
myApp (directory)
-app (directory)
--ui.R
--server.R
--global.R
--style.css
--mydata.xlsx
--mydata2.rds
--functions.R (contains functions I use in app)
-Dockerfile
-shiny-server.conf
-shiny-server.sh
I can get into my directory myApp and run shiny locally by doing runApp('app'). my shiny app runs perfectly.
However, when I try to build the image and run it, it give me error.
docker build -t myshinyapp_obs .
docker run -p 80:80 myshinyapp_obs
The application failed to start.
The application exited during initialization.
The docker image building process seems to be fine.
When run it on docker, I got error.
The interesting thing is when I simply copy any app from shiny gallery and put the ui.R and server.R file under the app folder, it works fine!!!
My question is why my app is not working? given the fact:
my app work perfectly locally
After copying shiny example from gallery into the app folder, the example app works fine.
How can that happen? I cannot figure it out. Spent hours trying to make it work but failed.
Below is my Dockfile:
# 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 \
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
# TODO: add further package if you need!
RUN R -e "install.packages(c('devtools','readxl','tidyverse','rlang','shiny','shinythemes', 'DT'), 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/
# Make the ShinyApp available at port 80
EXPOSE 80
# Copy further configuration files into the Docker image
COPY shiny-server.sh /usr/bin/shiny-server.sh
CMD ["/usr/bin/shiny-server.sh"]