Fix all shellcheck warnings

examples/docker-compose/bin/letsencrypt.sh

@@ -18,10 +18,10 @@ set -e
 
 # letsencrypt certificate server type (default is production).
 # Set `CERT_SERVER=--staging` for staging.
-: ${CERT_SERVER=''}
+: "${CERT_SERVER=''}"
 
 # Create Docker volume to contain the cert
-: ${SECRETS_VOLUME:=my-notebook-secrets}
+: "${SECRETS_VOLUME:=my-notebook-secrets}"
 docker volume create --name $SECRETS_VOLUME 1>/dev/null
 # Generate the cert and save it to the Docker volume
 docker run --rm -it \

examples/docker-compose/bin/vbox.sh

@@ -3,9 +3,9 @@
 # Distributed under the terms of the Modified BSD License.
 
 # Set reasonable default VM settings
-: ${VIRTUALBOX_CPUS:=4}
+: "${VIRTUALBOX_CPUS:=4}"
 export VIRTUALBOX_CPUS
-: ${VIRTUALBOX_MEMORY_SIZE:=4096}
+: "${VIRTUALBOX_MEMORY_SIZE:=4096}"
 export VIRTUALBOX_MEMORY_SIZE
 
 docker-machine create --driver virtualbox "$@"