chore: fix local docker docs build

Following #1499, conda is unable to find required version of sphinx:

```
0.098 + /opt/conda/bin/conda env create -n docs.clades.nextstrain.org
0.501 Channels:
0.501  - defaults
0.501 Platform: linux-64
0.501 Collecting package metadata (repodata.json): ...working... done
2.229 Solving environment: ...working... failed
2.439 
2.439 PackagesNotFoundError: The following packages are not available from current channels:
2.439 
2.439   - sphinx=7.2*
2.439 
2.439 Current channels:
2.439 
2.439   - https://repo.anaconda.com/pkgs/main/linux-64
2.439   - https://repo.anaconda.com/pkgs/r/linux-64
2.439 
2.439 To search for alternate channels that may provide the conda package you're
2.439 looking for, navigate to
2.439 
2.439     https://anaconda.org
2.439 
2.439 and use the search bar at the top of the page.
```


Let's try and add coda-forge channel

docs/Dockerfile

@@ -56,6 +56,7 @@ WORKDIR /workdir
 COPY environment.yml /workdir
 
 RUN set -euxo pipefail >/dev/null \
+  && conda config --add channels conda-forge \
   && conda env create -n "docs.clades.nextstrain.org"
 
 USER ${USER}