build-tools: base image Dockerfile: invalid apt clean command

[Davlet Panech]

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

build-tools/build-docker-images/stx-debian/Dockerfile.stable

The base image Dockerfile contains an invalid "rm" command:
   apt-get clean && \
   rm -rf /var/lib/apt/files/*

There's no /var/lib/apt/files in apt.

Severity
--------
Minor

Steps to Reproduce
------------------
Build the docker file

Expected Behavior
------------------
Docker build succeeds

Actual Behavior
----------------
Build succeeds, but apt cache is not completely cleaned between steps.

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

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

Branch/Pull Time/Commit
-----------------------
master/2022-07-25

Last Pass
---------
N/A

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

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

Workaround
----------
N/A

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/starlingx/root/+/850934

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/starlingx/root/+/850934
Committed: https://opendev.org/starlingx/root/commit/b4537736b245ee89de637fafc06ec55677801696
Submitter: "Zuul (22348)"
Branch: master

commit b4537736b245ee89de637fafc06ec55677801696
Author: Davlet Panech <email address hidden>
Date: Mon Jul 25 12:34:02 2022 -0400

    build-tools: fix invalid apt cache removal

    Fix Debian base image docker file attempting to remove a non-existent
    directory when cleaning apt cache.

    TESTS
    ===================================
    Build the base image

    Closes-Bug: 1982767
    Signed-off-by: Davlet Panech <email address hidden>
    Change-Id: Ibb786be00570b72dcbe52fd920cbec2e10661969