feat: support inline tag in image.repository for jupyterlab chart

Allow repository field to include a tag (e.g. "repo:tag"), in which case
the tag field is ignored. Update docs and custom-values to reflect the
new behavior and add dshm volume notes.

manifests/helm/jupyterlab/1.0.0/values.yaml

@@ -2,8 +2,9 @@
 # Deploys JupyterLab using pure Kubernetes resources without an Operator.
 
 # JupyterLab Docker image
-# Full image ref: <registry>/<repository>:<tag>
+# Full image ref: [registry/]repository[:tag]
 # Leave registry empty to use Docker Hub (default).
+# tag is optional if repository already contains a tag (e.g. "repo:tag").
 image:
   registry: "quay.io"
   repository: "jupyter/scipy-notebook"