Proposed improvements for G_SLICE (#1697)

* Try to fix mamba under aarch64 QEMU by setting ENV G_SLICE=always-malloc

* Set G_SLICE inside if statement

* Write "extract_threads: 1" to /opt/.condarc

Co-authored-by: Ayaz Salikhov <mathbunnyru@gmail.com>

datascience-notebook/Dockerfile

@@ -61,7 +61,13 @@ USER ${NB_UID}
 
 # R packages including IRKernel which gets installed globally.
 # r-e1071: dependency of the caret R package
-RUN mamba install --quiet --yes \
+RUN arch=$(uname -m) && \
+    if [ "${arch}" == "aarch64" ]; then \
+        # Prevent libmamba from sporadically hanging on arm64 under QEMU
+        # <https://github.com/mamba-org/mamba/issues/1611>
+        export G_SLICE=always-malloc; \
+    fi && \
+    mamba install --quiet --yes \
     'r-base' \
     'r-caret' \
     'r-crayon' \