Incremental docker image updates misconfigures container entrypoint and cmd

[Dan Voiculeasa]

Brief Description
-----------------

Severity
--------
Minor: System/Feature is usable with minor issue

Steps to Reproduce
------------------

Expected Behavior
------------------
${MY_REPO}/build-tools/build-docker-images/update-stx-image.sh keeps the original CMD and ENTRYPOINT of the container.

inspect:
"Cmd": [
    "/bin/bash"
],
"Entrypoint": null,

Since null cannot be generated the following entrypoint seems acceptable:
"Entrypoint": [
    ""
],

Actual Behavior
----------------
${MY_REPO}/build-tools/build-docker-images/update-stx-image.sh doesn't keep the original CMD and ENTRYPOINT of the container.

inspect:
"Cmd": [
    "/bin/sh",
    "-c",
    "/bin/bash"
],
"Entrypoint": [
    "/bin/bash"
],

Reproducibility
---------------
100%

System Configuration
--------------------
build-tools

Branch/Pull Time/Commit
-----------------------
Nov 2020

Last Pass
---------

Timestamp/Logs
--------------

Test Activity
-------------
Developer Testing

Workaround
----------

[Dan Voiculeasa (dvoicule)]

https://review.opendev.org/c/starlingx/root/+/765645

[Ghada Khalil (gkhalil)]

Fixed by: https://review.opendev.org/c/starlingx/root/+/765645
Merged on 2020-12-08