ERROR: (gcloud.app.deploy) You must provide your own Dockerfile when using a custom runtime

I am trying to schedule a r job on google cloud using app engine. I am following the article

https://code.markedmondson.me/4-ways-schedule-r-scripts-on-google-cloud-platform/

However, when I try to deploy the app using gcloud app deploy --project shiny-demo I get the following error

Enabling service [appengineflex.googleapis.com] on project [shiny-demo]...
Waiting for async operation operations/acf.33c5e6da-6f9e-4e66-b7d3-f0611eddc0ce to complete...
Operation finished successfully. The following command can describe the Operation details:
 gcloud services operations describe operations/tmo-acf.33c5e6da-6f9e-4e66-b7d3-f0611eddc0ce
Beginning deployment of service [default]...
ERROR: (gcloud.app.deploy) You must provide your own Dockerfile when using a custom runtime. Otherwise provide a "runtime" field with one of the supported runtimes.

I do have a dockerfile in the same folder as my yaml file, however somehow it does not recognize the dockerfile. Below is my yaml and docker files:

YAML
runtime: custom
env: flex

manual_scaling:
  instances: 1
resources:
  cpu: 1
  memory_gb: 0.5
  disk_size_gb: 10

env_variables:
  GCS_AUTH_FILE: authkey.json
DOCKER

FROM trestletech/plumber
LABEL maintainer="mark"
RUN export DEBIAN_FRONTEND=noninteractive; apt-get -y update 
 && apt-get install -y libcairo2-dev 
    libcurl4-openssl-dev 
    libgmp-dev 
    libpng-dev 
    libssl-dev 
    libxml2-dev 
    make 
    pandoc 
    pandoc-citeproc 
    zlib1g-dev
RUN ["install2.r", "-r 'https://cloud.r-project.org'", "readr", "googleCloudStorageR", "bigrquery", "Rcpp", "digest", "crayon", "withr", "mime", "R6", "jsonlite", "xtable", "magrittr", "httr", "curl", "testthat", "devtools", "hms", "shiny", "httpuv", "memoise", "htmltools", "openssl", "tibble", "remotes"]
RUN ["installGithub.r", "MarkEdmondson1234/[email protected]", "hadley/[email protected]"]
WORKDIR /payload/
COPY [".", "./"]

EXPOSE 8080
ENTRYPOINT ["R", "-e", "pr <- plumber::plumb(commandArgs()[4]); pr$run(host='0.0.0.0', port=8080)"]
CMD ["myscript.R"]

I am a bit new to dockers, so I really hope someone could please help me fix this issue.

Thanks in advance !!

Source: StackOverflow