Fix for granting SUDO to jovyan user and run bash commands.

Test image builded with:
> docker build -t $USER/base-notebook -f Dockerfile .

> docker run -ti --rm -p 8888:8888 -e GRANT_SUDO=yes --user root jupyter/base-notebook:033056e6 bash
root@84db8819258d:~#

docker run -ti --rm -p 8888:8888 -e GRANT_SUDO=yes --user root $USER/base-notebook bash
> root@c23f1fd7d279:~#

> docker run -ti --rm -p 8888:8888 -e GRANT_SUDO=yes --user root jupyter/base-notebook:033056e6 start.sh
Set username to: jovyan
usermod: no changes
Set jovyan GID to: 100
Granting jovyan sudo access
Execute the command:
HOSTNAME=a231202e6d1a
TERM=xterm
SHELL=/bin/bash
NB_USER=jovyan
LC_ALL=en_US.UTF-8
USER=jovyan
MAIL=/var/mail/jovyan
PATH=/opt/conda/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
MINICONDA_VERSION=4.3.30
PWD=/home/jovyan
LANG=en_US.UTF-8
GRANT_SUDO=yes
HOME=/home/jovyan
SHLVL=1
LANGUAGE=en_US.UTF-8
no_proxy=*.local, 169.254/16
LOGNAME=jovyan
DEBIAN_FRONTEND=noninteractive
CONDA_DIR=/opt/conda
NB_GID=100
NB_UID=1000
_=/usr/bin/env
tlinnet@linmac:base-notebook$

> docker run -ti --rm -p 8888:8888 -e GRANT_SUDO=yes --user root $USER/base-notebook start.sh
Set username to: jovyan
usermod: no changes
Set jovyan GID to: 100
Granting jovyan sudo access
Execute the command:
No arguments supplied
HOSTNAME=364e5fc77224
SHELL=/bin/bash
TERM=xterm
LC_ALL=en_US.UTF-8
USER=jovyan
SUDO_USER=root
SUDO_UID=0
USERNAME=jovyan
MAIL=/var/mail/jovyan
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/snap/bin
PWD=/home/jovyan
LANG=en_US.UTF-8
SHLVL=1
SUDO_COMMAND=/bin/bash -c env; PATH=$PATH; bash
HOME=/home/jovyan
LANGUAGE=en_US.UTF-8
LOGNAME=jovyan
SUDO_GID=0
_=/usr/bin/env
jovyan@364e5fc77224:~$

> docker run -ti --rm -p 8888:8888 -e GRANT_SUDO=yes --user root jupyter/base-notebook:033056e6 start.sh echo hello
tlinnet@linmac:base-notebook$ docker run -ti --rm -p 8888:8888 -e GRANT_SUDO=yes --user root jupyter/base-notebook:033056e6 start.sh echo hello
Set username to: jovyan
usermod: no changes
Set jovyan GID to: 100
Granting jovyan sudo access
Execute the command: echo hello
hello
tlinnet@linmac:base-notebook$

> docker run -ti --rm -p 8888:8888 -e GRANT_SUDO=yes --user root $USER/base-notebook start.sh echo hello
Set username to: jovyan
usermod: no changes
Set jovyan GID to: 100
Granting jovyan sudo access
Execute the command: echo hello
hello
tlinnet@linmac:base-notebook$

base-notebook/start.sh

@@ -47,7 +47,12 @@ if [ $(id -u) == 0 ] ; then
 
     # Exec the command as NB_USER
     echo "Execute the command: $*"
-    exec su $NB_USER -c "env PATH=$PATH $*"
+    if [ $# -eq 0 ]; then
+        echo "No arguments supplied"
+        sudo -H -u $NB_USER bash -c 'env; PATH=$PATH; bash'
+    else
+        exec su $NB_USER -c "env PATH=$PATH $*"
+    fi
 else
   if [[ ! -z "$NB_UID" && "$NB_UID" != "$(id -u)" ]]; then
       echo 'Container must be run as root to set $NB_UID'