Containerize: accommodate nested or pre-existing spack-env paths (#41558)

The current `mkdir {{ paths.environment }}` will generate an error if:
* `{{ paths.environment }}` already exists, or
* `{{ paths.environment }}` is nested in non-existing dirs.

Adding `-p` to the command will make this robust to both possibilities.

Set noclobber bash option when writing manifest.

share/spack/templates/container/Dockerfile

@@ -16,7 +16,8 @@ RUN {% if os_package_update %}{{ os_packages_build.update }} \
 
 # What we want to install and how we want to install it
 # is specified in a manifest file (spack.yaml)
-RUN mkdir {{ paths.environment }} \
+RUN mkdir -p {{ paths.environment }} && \
+set -o noclobber \
 {{ manifest }} > {{ paths.environment }}/spack.yaml
 
 # Install the software, remove unnecessary deps