fix(Dockerfile): remove hardcoded JAVA_HOME arch so image works on arm64 and amd64

[sallycr]

Is your feature request related to a problem? Please describe.

The example Dockerfile sets `ENV JAVA_HOME=/usr/lib/jvm/java-11-openjdk-${TARGETARCH}` where `TARGETARCH` defaults to `amd64`. On arm64 k3d nodes (Apple Silicon), this path does not exist, causing `spark-submit` to fail immediately:

```
/usr/lib/jvm/java-11-openjdk-amd64/bin/java: No such file or directory
```

The Dockerfile already auto-detects the correct Java path at build time and writes it to `/etc/environment`:

```dockerfile
RUN JAVA_HOME=$(dirname $(dirname $(readlink -f $(which java)))) &&
echo "JAVA_HOME=${JAVA_HOME}" >> /etc/environment
```

But this is immediately overridden by the subsequent `ENV JAVA_HOME=...` line. The current workaround is to source `/etc/environment` in `entrypoint.sh` before invoking `spark-submit`, but this is a bandaid — the root cause is the hardcoded arch in the `ENV` line.

Describe the solution you'd like

Remove the hardcoded `ENV JAVA_HOME=/usr/lib/jvm/java-11-openjdk-${TARGETARCH}` line from `examples/Dockerfile` and replace it with a single `RUN` step that auto-detects and exports `JAVA_HOME` for any architecture:

```dockerfile
RUN JAVA_HOME=$(dirname $(dirname $(readlink -f $(which java)))) &&
echo "export JAVA_HOME=${JAVA_HOME}" >> /etc/profile.d/java.sh &&
echo "JAVA_HOME=${JAVA_HOME}" >> /etc/environment
ENV JAVA_HOME=""
```

Or use a Docker build arg with auto-detection that is evaluated at build time rather than as a static string.

Once this is fixed, the `entrypoint.sh` workaround (sourcing `/etc/environment`) can also be removed.

Describe alternatives you've considered

• Keep the `entrypoint.sh` workaround (current state) — works but is fragile and non-obvious
• Use `--build-arg TARGETARCH=arm64` explicitly — puts the burden on the caller
• Switch the base image to one where Java is already on `PATH` without `JAVA_HOME`

Additional context

• Introduced as a workaround in PR feat(example): California Housing XGBoost end-to-end UniFlow workflow #1285 (canvasflex/california-housing-example)
• Affects any developer running the example on Apple Silicon with a k3d sandbox
• Related: `entrypoint.sh` change in PR feat(example): California Housing XGBoost end-to-end UniFlow workflow #1285 that sources `/etc/environment` as the short-term fix