docker: add README.md

Problem: there is no README for the docker containers.

Add one.

Author: jameshcorbett

src/test/docker/README.md

@@ -0,0 +1,50 @@
+### Docker Images for SCR
+
+The Dockerfiles, resulting docker images, and `docker-run-checks.sh`
+script contained herein are used as part of the strategy for CI testing
+of SCR.
+
+Docker is used under CI to speed up deployment of an
+environment with correct build dependencies and to keep a docker
+image deployed at `libscr/scr` DockerHub with latest master build
+(`libscr/scr:latest`) and tagged builds (`scr/SCR:v<tag>`),
+which can be used by other projects to build against the latest
+or a tagged version of SCR.
+
+#### libscr/scr Docker Images
+
+The Dockerfiles `fedora40/Dockerfile`,
+`el9/Dockerfile`, and `alpine/Dockerfile` describe the images built
+under `libscr/scr:fedora40`, `libscr/scr:el9`, and `libscr/scr:alpine`
+respectively, and include the base dependencies required to build SCR.
+The Dockerfiles should be kept up to date for a single point of management.
+
+#### Multiarch Images
+
+Building the images for linux/amd64 and linux/arm64 requires the
+Docker buildx extensions, see
+
+ https://www.docker.com/blog/multi-arch-build-and-images-the-simple-way/
+
+and run
+```
+$  docker buildx build --push --platform=linux/arm64,linux/amd64 --tag libscr/scr:el9 -f src/test/docker/el9/Dockerfile .
+```
+
+to build and push images to docker hub.
+
+#### Local Testing
+
+Developers can test the docker images themselves. If new dependencies are needed,
+they can update the `$image` Dockerfiles manually.
+To create a local Docker image, run the command:
+
+```
+docker build -t libscr/scr:$image src/test/docker/$image
+```
+
+To test the locally created image, run:
+
+```
+src/test/docker/docker-run-checks.sh -i $image [options] -- [arguments]
+```