How to define a variable in a Dockerfile?

You can use ARG – see https://docs.docker.com/engine/reference/builder/#arg

The ARG instruction defines a variable that users can pass at
build-time to the builder with the docker build command using the
--build-arg <varname>=<value> flag. If a user specifies a build
argument that was not defined in the Dockerfile, the build outputs an
error.

Can be useful with COPY during build time (e.g. copying tag specific content like specific folders)
For example:

ARG MODEL_TO_COPY
COPY application ./application
COPY $MODEL_TO_COPY ./application/$MODEL_TO_COPY

While building the container:

docker build --build-arg MODEL_TO_COPY=model_name -t <container>:<model_name specific tag> .