[MINOR] Fix Dockerfile

Author: jongyoul

docs/development/contribution/how_to_contribute_code.md

@@ -54,10 +54,10 @@ Get the source code on your development machine using git.
 git clone git://gitbox.apache.org/repos/asf/zeppelin.git zeppelin
 ```
 
-You may also want to develop against a specific branch. For example, for branch-0.11.0
+You may also want to develop against a specific branch. For example, for branch-0.12
 
 ```bash
-git clone -b branch-0.11.0 git://gitbox.apache.org/repos/asf/zeppelin.git zeppelin
+git clone -b branch-0.12 git://gitbox.apache.org/repos/asf/zeppelin.git zeppelin
 ```
 
 Apache Zeppelin follows [Fork & Pull](https://github.com/sevntu-checkstyle/sevntu.checkstyle/wiki/Development-workflow-with-Git:-Fork,-Branching,-Commits,-and-Pull-Request) as a source control workflow.

scripts/docker/zeppelin/bin/Dockerfile

@@ -51,8 +51,11 @@ RUN set -ex && \
     export PATH=/opt/conda/bin:$PATH && \
     conda config --set always_yes yes --set changeps1 no && \
     conda info -a && \
+    conda config --remove channels defaults && \
+    conda config --add channels conda-forge && \
+    conda config --set channel_priority strict && \
     conda install mamba -c conda-forge && \
-    mamba env update -f /env_python_3_with_R.yml --prune && \
+    mamba env create -f /env_python_3_with_R.yml -c conda-forge && \
     # Cleanup
     rm -v miniconda.sh anaconda.sha256  && \
     # Cleanup based on https://github.com/ContinuumIO/docker-images/commit/cac3352bf21a26fa0b97925b578fb24a0fe8c383

scripts/docker/zeppelin/bin/env_python_3_with_R.yml

@@ -1,9 +1,8 @@
 name: python_3_with_R
 channels:
   - conda-forge
-  - defaults
 dependencies:
-  - python=3.7
+  - python=3.9
   - pycodestyle
   - scipy
   - numpy=1.19.5