Improve DBSClient installation inside WMCore docker images

[mapellidario]

Impact of the new feature

All the wmcore services that rely on dbsclient

Is your feature request related to a problem? Please describe.

This issue is a followup of #12097 .

The image build by Alan registry.cern.ch/cmsweb/reqmgr2ms-output:2.3.6rc5 and deployed to cmsweb-testbed contains the correct wmcore version (2.3.6rc5) [1] but the dbsclient version is not what we desire ( 4.0.19 ) [2]

Describe the solution you'd like

We should be able to get into wmcore images the exact dbs-client version that we ask for. We should use the == operator in our requirements.txt instead of ~=.

We can always switch back to ~= if we see that pip can not deal with the fixed "patch" versions for dbs-client and needs some wiggle room

Moreover, we should install dbs-client without specifying the desired version, such as in ms-unmerged

Describe alternatives you've considered

We could change the minor version of the dbsclient and continue to use ~=, but since this is a bugfix and did not add any new feature, it would against the semantic versioning rationale

---

[1]

_reqmgr2ms@ms-output-6b776bb56c-nwfkk:/data$ cat /usr/local/lib/python3.8/site-packages/WMCore/__init__.py
[...]
__version__ = '2.3.6rc5'

[2]

_reqmgr2ms@ms-output-6b776bb56c-nwfkk:/data$ ls /usr/local/lib/python3.8/site-packages/dbs*
[...]
/usr/local/lib/python3.8/site-packages/dbs3_client-4.0.12.dist-info:

[amaltaro]

Fixed by #12102