Add README

docker/README.md

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+# Docker
+
+## Using docker-compose
+
+You may build and test your changes by using the compose file:
+
+    docker-compose -f ./docker/docker-compose.yml up
+
+## Entering the build environment
+
+While in the repository root, to enter the environment and be presented with a
+shell:
+
+    docker run -it --rm -u builder \
+      -v $PWD:/workspace ghcr.io/KaruroChori/vs-fltk:build-env
+
+This will mount your current directory as */workspace* inside the container.
+Your username will be *builder*. By default, you will not have root privileges
+(which are not necessary to build and test). However, you can use `sudo` if
+you need to run `apt` or any other commands that require root access.
+
+## Getting the image
+
+You can pull the image manually:
+
+    docker pull ghcr.io/KaruroChori/vs-fltk:build-env
+
+If you use `docker-compose` or `docker run ...` the image will be pulled
+automatically the first time.
+
+