Add build instructions to README

README.md

@@ -1,2 +1,52 @@
 # bmi-example-cxx-docker
-The C++ BMI example, Dockerized
+
+The [C++ example](https://github.com/csdms/bmi-example-c)
+for the [Basic Model Interface](https://bmi.readthedocs.io),
+dockerized.
+
+## Build the image
+
+Build this example locally with:
+```
+docker build --tag bmi-example-cxx .
+```
+The image is based on the [csdms/bmi](https://hub.docker.com/r/csdms/bmi) image.
+The OS is Linux/Ubuntu.
+`conda` and `mamba` are installed in `CONDA_DIR=/opt/conda`,
+and the *base* environment is activated.
+The C++ BMI specification and example, including the `run_bmiheatc` executable, are installed into it.
+
+## Run a container
+
+Run a container from this image interactively:
+```
+docker run -it bmi-example-cxx
+```
+This starts a bash shell in the container.
+Run the example program `run_bmiheatcxx` with a configuration file:
+```bash
+cd /tmp
+echo "1.5, 8.0, 6, 5" > config.txt
+run_bmiheatcxx config.txt
+```
+View the program's output with:
+```bash
+cat bmiheatcxx.out
+```
+
+## Developer notes
+
+A versioned, multiplatform image built from this repository is hosted on Docker Hub
+at [csdms/bmi-example-cxx](https://hub.docker.com/r/csdms/bmi-example-cxx/).
+When this repository is tagged,
+an image is automatically built and pushed to Docker Hub
+by the [release](./.github/workflows/release.yml) CI workflow.
+To manually build and push an update, run:
+```
+docker buildx build --platform linux/amd64,linux/arm64 -t csdms/bmi-example-cxx:latest --push .
+```
+A user can pull this image from Docker Hub with:
+```
+docker pull csdms/bmi-example-cxx
+```
+optionally with the `latest` tag or with a version tag.