build-tools: image update script corrupts docker entrypoint commands

[Davlet Panech]

Brief Description
-----------------
The script sometimes corrupts ENTRYPOINT and CMD entries in the images being updated. The logic that parses these entries in "docker image inspect" output seems incorrect: https://opendev.org/starlingx/root/src/commit/c2d62e59173832e4597249817b891b990f5d6600/build-tools/build-docker-images/update-stx-image.sh#L456

Observed with starlingx/rvmc image. Original image had:

            "Entrypoint": [
                "/bin/sh",
                "-c",
                "/usr/local/bin/rvmc.py --target $target --debug $debug"
            ],

But after modification it becomes

            "Entrypoint": [
                "/bin/sh",
                "-c",
                "/usr/local/bin/rvmc.py",
                "--target",
                "$target",
                "--debug",
                "$debug"
            ],

A similar problem exists with "CMD" .

Severity
--------
Major

Steps to Reproduce
------------------
See above

Expected Behavior
------------------
See above

Actual Behavior
----------------
See above

Reproducibility
---------------
Reproducible

System Configuration
--------------------
N/A

Branch/Pull Time/Commit
-----------------------
master/2023-10-04

Last Pass
---------
None

Timestamp/Logs
--------------
N/A

Test Activity
-------------
N/A

Workaround
----------
Correct entrypoint manually using "docker run/docker commit"