I'm trying to create a temporary folder and then set the path as an environment variable for use in later Dockerfile instructions:
FROM alpine
RUN export TEMPLATES_DIR=$(mktemp -d)
ENV TEMPLATES_DIR=$TEMPLATES_DIR
RUN echo $TEMPLATES_DIR
Above is what I've tried, any idea how I can achieve this?
Anything you run in a Dockerfile will be persisted forever in the resulting Docker image. As a general statement, you don't need to use environment variables to specify filesystem paths, and there's not much point in creating "temporary" paths. Just pick a path; it doesn't even need to be a "normal" Linux path since the container filesystem is isolated.
RUN mkdir /templates
It's common enough for programs to use environment variables for configuration (this is a key part of the "12-factor" design) and so you can set the environment variable to the fixed path too
ENV TEMPLATES_DIR=/templates
In the sequence you show, every RUN step creates a new container with a new shell, and so any environment variables you set in a RUN command get lost at the end of that step. You can't set a persistent environment variable in quite the way you're describing; Create dynamic environment variables at build time in Docker discusses this further.
If it's actually a temporary directory, and you're intending to clean it up, there are two more possibilities. One is to do all of the work you need inside a single RUN step that runs multiple commands. The environment variable won't outlive that RUN step, but it will be accessible within it.
RUN export TEMPLATES_DIR=$(mktemp -d) \
&& echo "$TEMPLATES_DIR" \
&& rm -rf "$TEMPLATES_DIR"
A second is to use a multi-stage build to do your "temporary" work in one image, but then copy the "permanent" parts of it out of that image into the final image you're actually shipping.
