Docker container with node, python and external libs

I have an application that consists of a web server, with node.js + Express, that also utilizes the file system, that calls external python programs.

The python programs are utilizing external packages (FreeCAD) packages.

I want to run everything inside a docker container.

For two days, i have been reading stuff and i have to say docker is very confusing.

I found examples of standalone node programs, or standalone python programs.

But what is the best approach to run my application inside a container?

Do i use a Dockerfile, in order to pull the neccesary docker images and then i configure them?

Or do i have to dockerize my existing application?

EDIT: My current DockerFile:

FROM ubuntu
RUN DEBIAN_FRONTEND=noninteractive apt-get update
RUN DEBIAN_FRONTEND=noninteractive apt-get install -y software-properties-common

#install freecad
RUN add-apt-repository ppa:freecad-maintainers/freecad-daily
RUN DEBIAN_FRONTEND=noninteractive apt-get update
RUN DEBIAN_FRONTEND=noninteractive apt-get install -y freecad-daily