Fix custom conda environment recipe

docs/using/recipe_code/custom_environment.dockerfile

@@ -0,0 +1,33 @@
+FROM jupyter/base-notebook
+
+# name your environment and choose the python version
+ARG env_name=python38
+ARG py_ver=3.8
+
+# you can add additional libraries here
+RUN mamba create --yes -p "${CONDA_DIR}/envs/${env_name}" \
+    python=${py_ver} \
+    'ipykernel' \
+    'jupyterlab' && \
+    mamba clean --all -f -y
+
+# alternatively, you can comment out the lines above and uncomment those below
+# if you'd prefer to use a YAML file present in the docker build context
+
+# COPY --chown=${NB_UID}:${NB_GID} environment.yml /tmp/
+# RUN mamba env create -p "${CONDA_DIR}/envs/${env_name}" -f /tmp/environment.yml && \
+#     mamba clean --all -f -y
+
+# create Python kernel and link it to jupyter
+RUN "${CONDA_DIR}/envs/${env_name}/bin/python" -m ipykernel install --user --name="${env_name}" && \
+    fix-permissions "${CONDA_DIR}" && \
+    fix-permissions "/home/${NB_USER}"
+
+# any additional `pip` installs can be added by using the following line
+# using `mamba` is highly recommended
+RUN "${CONDA_DIR}/envs/${env_name}/bin/pip" install --no-cache-dir \
+    'flake8'
+
+# if you do not want this environment to be the default one, comment this line
+# hadolint ignore=DL3059
+RUN echo "conda activate ${env_name}" >> "${HOME}/.bashrc"