The Dockerfile is simply a text document, containing all the commands that would be issued on the
command-line to build an image - in short, the
Dockerfile defines an environment.
The Dockerfile is built automatically, and populated with appropriate commands (see below).
However, you can also add any commands you wish to the
Dockerfile, for example to install
system packages, or to configure the environment.
Dockerfile executes its commands in sequence. This means that commands to install Node
(for example) must come before commands to run Node packages.