Error Building Custom MPI Image Following Documentation

Hello team,

I followed the documentation to run the PI example, but when I tried to build my own images based on the docs at https://github.com/kubeflow/mpi-operator/tree/master/examples/v2beta1/pi, I encountered the following error:

```
docker build -t mpi-pi .
[+] Building 0.0s (2/2) FINISHED docker:default
=> [internal] load build definition from Dockerfile 0.0s
=> => transferring dockerfile: 257B 0.0s
=> [internal] load .dockerignore 0.0s
=> => transferring context: 2B 0.0s
Dockerfile:8
--------------------
6 | RUN mpic++ /src/pi.cc -o /pi
7 |
8 | >>> FROM mpioperator/openmpi:${BASE_LABEL}
9 |
10 | COPY --from=builder /pi /home/mpiuser/pi
--------------------
ERROR: failed to solve: failed to parse stage name "mpioperator/openmpi:": invalid reference format
```

#### Question

Does it make sense to have the `ARG BASE_LABEL` in this image? It seems that the `${BASE_LABEL}` variable is not being resolved, causing the build to fail.

#### Additional Information

- I'm using the latest version of Docker
- I'm following the example Dockerfile provided in the documentation

Any assistance or clarification would be greatly appreciated. Thank you!

Start with the Dockerfile in examples/v2beta1/pi and the linked documentation, then reproduce the reported docker build command to inspect how BASE_LABEL is declared and used. Done means the documented custom image build resolves its base image and completes successfully, or the example clearly explains the required value.

Written by the indexing model from the issue text.