Pin MongoDB Docker image to mongo:8

[Copilot]

PyMongo requires MongoDB 4.2+ (wire protocol v8), but the cached mongo:latest image in CI was resolving to an older version, causing test failures across all MongoDB backend runs.

Changes

• CI workflow (.github/workflows/ci-test.yml): mongo:latest → mongo:8
• Local dev scripts (scripts/test-local.sh, scripts/docker-compose.all-cores.yml, Makefile): same pin
• Docs (README.rst, scripts/README-local-testing.md): updated example commands to match

Original prompt

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This section details on the original issue you should resolve

<issue_title>Upgrade CI Mongo server version</issue_title>
<issue_description>When testing async Mongo caching on CI, I end up with this error:

E           pymongo.errors.ConfigurationError: Server at 127.0.0.1:27019 reports wire version 7, but this version of PyMongo requires at least 8 (MongoDB 4.2).

Report example:

• https://github.com/python-cachier/cachier/actions/runs/22005125468/job/63587027714?pr=334

Can we upgrade the Mongo server version in the CI environment? </issue_description>

<agent_instructions>Please open a PR handling this by pinning the CI's docker to mongo:8 (not 8.0).</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@shaypal5 We're using the following code to start MongoDB in Docker: ```yaml - name: Start MongoDB in docker if: matrix.backend == 'mongodb' run: | # start MongoDB in a container docker run -d -p ${{ env.CACHIER_TEST_PORT }}:27017 --name mongodb mongo:latest # wait for MongoDB to start, which is in average 5 seconds sleep 5 # show running containers docker ps -a ```

Doesn't this mean we're already using the latest Docker image for MongoDB?</comment_new>
<comment_new>@shaypal5
Isn't it better to just manually invalidate/refresh the cache explicitly? @gencurrent </comment_new>
<comment_new>@shaypal5
@copilot what do you think? Can we have something to refresh the docker images used in our CI once a month or something? Like a monthly workflow? Or in the regular CI, something that refreshes the cache on the first of every month?</comment_new>
<comment_new>@shaypal5
Oh, ok. I get it. We actually WANT to pin MongoDB to 8.0 in the CI.</comment_new>

• Fixes Upgrade CI Mongo server version #335

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.94%. Comparing base (2986978) to head (1f59045).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #370   +/-   ##
=======================================
  Coverage   99.94%   99.94%           
=======================================
  Files          12       12           
  Lines        1735     1735           
  Branches      218      218           
=======================================
  Hits         1734     1734           
  Partials        1        1           

Flag	Coverage Δ
local	59.30% <ø> (ø)
mongodb	40.69% <ø> (ø)
postgres	43.40% <ø> (ø)
redis	46.80% <ø> (ø)
s3	42.47% <ø> (ø)

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