New docker build and pin old hvplot

[tlvu]

Pin hvplot for https://github.com/holoviz/hvplot/issues/498 (violin plot not working in latest hvplot 0.6.0).

Jenkins successful build: http://jenkins.ouranos.ca/job/PAVICS-e2e-workflow-tests/job/new-docker-build-and-pin-pytest/4/console (with pavics-sdi PR https://github.com/Ouranosinc/pavics-sdi/pull/173).

Noticeable changes:

<   - hvplot=0.6.0=pyh9f0ad1d_0
>   - hvplot=0.5.2=py_0

<   - dask=2.20.0=py_0
>   - dask=2.22.0=py_0

<   - geopandas=0.8.0=py_1
>   - geopandas=0.8.1=py_0

<   - pandas=1.0.5=py37h0da4684_0
>   - pandas=1.1.0=py37h3340039_0

<   - matplotlib=3.2.2=1
>   - matplotlib=3.3.0=1

<   - numpy=1.18.5=py37h8960a57_0
>   - numpy=1.19.1=py37h8960a57_0

<   - cryptography=2.9.2=py37hb09aad4_0
>   - cryptography=3.0=py37hb09aad4_0

<   - python=3.7.6=h8356626_5_cpython
>   - python=3.7.8=h6f2ec95_1_cpython 

<   - nbval=0.9.5=py_0
>   - nbval=0.9.6=pyh9f0ad1d_0

<   - pytest=5.4.3=py37hc8dfbb8_0
>   - pytest=6.0.1=py37hc8dfbb8_0

Full conda env export diff: 200716-200803-conda-env-export.diff.txt

Full new conda env export (200803): 200803-conda-env-export.yml.txt

Full new conda env export (200730 not final new image of this PR): 200730-conda-env-export.yml.txt