fix: docker tests in cicd

[dee-me-tree-or-love]

What

Fixes #439. The docker image could not be built due to incompatibility of gevent==21.1.2 with the newer version of Cython installed as part of the alpine packages during the build. See related releases (21.8.0 and 21.12.0) at https://www.gevent.org/changelog.html#id1.

Original error

Example failing workflow (here)

  Error compiling Cython file:
  ------------------------------------------------------------
  ...
  cdef load_traceback
  cdef Waiter
  cdef wait
  cdef iwait
  cdef reraise
  cpdef GEVENT_CONFIG
        ^
  ------------------------------------------------------------
  
  src/gevent/_gevent_cgreenlet.pxd:182:6: Variables cannot be declared with 'cpdef'. Use 'cdef' instead.

Related info

• See the issue discussed at gevent: Cython 3.0.0a8 dropped support in cpdef gevent/gevent#1801