Nightly Docker build breaks on lmi.Dockerfile's djl_converter_wheel ARG

## Nightly Docker build breaks on `lmi.Dockerfile`'s `djl_converter_wheel` ARG

`serving/docker/lmi.Dockerfile` has this line:

```dockerfile
ARG djl_converter_wheel="https://publish.djl.ai/djl_converter/djl_converter-${djl_version//-*/}-py3-none-any.whl"
```

`${djl_version//-*/}` is bash syntax meant to strip a `-SNAPSHOT` suffix off the version (e.g. `0.36.0-SNAPSHOT` -> `0.36.0`). But Docker's `ARG` interpolation doesn't support this kind of pattern substitution at all - it only supports a small subset of shell parameter expansion (things like `${var:-default}`).

Tried it directly:

```
$ docker build --build-arg djl_version=0.36.0-SNAPSHOT ...
ERROR: failed to solve: failed to process "...${djl_version//-*/}...":
unsupported modifier (/) in substitution
```

I also tried `${djl_version%%-*}` (a different suffix-stripping syntax) and got the identical error - Docker just doesn't support any pattern-substitution operator here, full stop.

### Why this matters

Looking at `.github/workflows/docker-nightly-publish.yml`, the nightly build path (`mode != "release"`) sets:

```
export BUILD_VERSION_ARG_DJL="${DJL_VERSION}-SNAPSHOT"
```

and passes that straight into the Dockerfile as `--build-arg djl_version=...`. So the nightly build path always hits exactly the broken case above - `lmi.Dockerfile` would fail to build with this "unsupported modifier" error whenever it's built with a `-SNAPSHOT` version.

The release path (no `-SNAPSHOT` suffix) doesn't hit this, since the substitution is a no-op when there's no `-` in the version - which is probably why it's gone unnoticed.

### Suggested fix

The suffix-stripping needs to happen in real bash *before* the value is handed to Docker, not inside the Dockerfile's `ARG` line. For example, in `docker-nightly-publish.yml`, compute the stripped version separately:

```bash
DJL_CONVERTER_VERSION="${DJL_VERSION}" # no -SNAPSHOT suffix, just the plain version
```

and pass that as its own build-arg, or pass it through as-is and have the Dockerfile use a plain `${djl_version}` with no substitution, relying on the caller to always pass an already-clean version.

Happy to open a PR for this if that's useful - wanted to flag it as its own issue since it's unrelated to other work I've got in flight (#3067).

Start with serving/docker/lmi.Dockerfile and .github/workflows/docker-nightly-publish.yml; compare how djl_version and BUILD_VERSION_ARG_DJL are passed on the nightly path. Run the affected Docker build with a -SNAPSHOT version. Done means the nightly lmi.Dockerfile build no longer uses unsupported ARG substitution and succeeds with the intended converter version.

Written by the indexing model from the issue text.