docker: use devel version of panel to hopefully fix Panel preview pro…

…blem

See comment holoviz/panel#3440 (comment)

To hopefully fix this error:
```
ServerApp - ERROR - Uncaught exception GET
/panel-preview/render/workdir/PAVICS-e2e-workflow-tests/binder/test-notebooks/TEST-panel-servable.ipynb
(10.10.10.7)
HTTPServerRequest(protocol='http', host='lvu.ouranos.ca:8890',
method='GET',
uri='/panel-preview/render/workdir/PAVICS-e2e-workflow-tests/binder/test-notebooks/TEST-panel-servable.ipynb',
version='HTTP/1.1', remote_ip='10.10.10.7')
Traceback (most recent call last):
  File
"/opt/conda/envs/birdy/lib/python3.8/site-packages/tornado/web.py", line
1704, in _execute
    result = await result
  File
"/opt/conda/envs/birdy/lib/python3.8/site-packages/panel/io/jupyter_server_extension.py",
line 116, in get
    page = server_html_page_for_session(
  File
"/opt/conda/envs/birdy/lib/python3.8/site-packages/bokeh/embed/server.py",
line 255, in server_html_page_for_session
    bundle = bundle_for_objs_and_resources(None, resources)
  File
"/opt/conda/envs/birdy/lib/python3.8/site-packages/bokeh/embed/bundle.py",
line 211, in bundle_for_objs_and_resources
    js_files.extend(js_resources.js_files)
  File
"/opt/conda/envs/birdy/lib/python3.8/site-packages/panel/io/resources.py",
line 299, in js_files
    dist_dir = self.dist_dir
  File
"/opt/conda/envs/birdy/lib/python3.8/site-packages/panel/io/resources.py",
line 259, in dist_dir
    if self.absolute:
AttributeError: 'Resources' object has no attribute 'absolute'
```

docker/environment.yml

@@ -5,7 +5,7 @@ channels:
   - cdat
   - bokeh
   - plotly  # for jupyter-dash
-  - pyviz/label/dev  # for jupyter-panel-proxy
+  - pyviz/label/dev  # for jupyter-panel-proxy, panel
   - defaults
 
 dependencies:
@@ -58,7 +58,7 @@ dependencies:
   - jupyter_bokeh
   - pscript
   - h5netcdf
-  - panel
+  - panel >= 0.13.1a2
   # https://github.com/holoviz/panel
   - pyviz_comms  # (was labextension pyviz/jupyterlab_pyviz in jupyterlab v2)
   - holoviews