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 atbuild-time to the builder with the docker build command using the--build-arg <varname>=<value> flag. If a user specifies a buildargument that was not defined in the Dockerfile, the build outputs anerror.

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

ARG MODEL_TO_COPYCOPY application ./applicationCOPY $MODEL_TO_COPY ./application/$MODEL_TO_COPY

While building the container:

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

To answer your question:

In my Dockerfile, I would like to define variables that I can use later in the Dockerfile.

You can define a variable with:

ARG myvalue=3

Spaces around the equal character are not allowed.

And use it later with:

RUN echo $myvalue > /test

To my knowledge, only ENV allows that, as mentioned in "Environment replacement"

Environment variables (declared with the ENV statement) can also be used in certain instructions as variables to be interpreted by the Dockerfile.

They have to be environment variables in order to be redeclared in each new containers created for each line of the Dockerfile by docker build.

In other words, those variables aren't interpreted directly in a Dockerfile, but in a container created for a Dockerfile line, hence the use of environment variable.

This day, I use both ARG (docker 1.10+, and docker build --build-arg var=value) and ENV.
Using ARG alone means your variable is visible at build time, not at runtime.

My Dockerfile usually has:

ARG varENV var=${var}

In your case, ARG is enough: I use it typically for setting http_proxy variable, that docker build needs for accessing internet at build time.