docker-ci: upgrade pip version in the bash script

The CI fails with the error

```
EnvCommandError

  Command ['/usr/bin/python3', '-m', 'pip', 'install', '--no-deps', '-U', '/root/.cache/pypoetry/artifacts/07/6f/ab/ca33bde7c6751a5ad8d13495b766891cd70e61786112885733ce9b0562/cryptography-36.0.2-cp36-abi3-manylinux_2_24_x86_64.whl'] errored with the following return code 1, and output: 
  ERROR: cryptography-36.0.2-cp36-abi3-manylinux_2_24_x86_64.whl is not a supported wheel on this platform.
  

  at ~/.local/lib/python3.8/site-packages/poetry/utils/env.py:1195 in _run
      1191│                 output = subprocess.check_output(
      1192│                     cmd, stderr=subprocess.STDOUT, **kwargs
      1193│                 )
      1194│         except CalledProcessError as e:
    → 1195│             raise EnvCommandError(e, input=input_)
      1196│ 
      1197│         return decode(output)
      1198│ 
      1199│     def execute(self, bin, *args, **kwargs):
```

The solution is to upgrade the pip version as suggested in python-poetry/poetry#2688 (comment)

Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>x

contrib/docker/scripts/build.sh

@@ -21,15 +21,20 @@ export VALGRIND=${VALGRIND:-0}
 export FUZZING=${FUZZING:-0}
 export LIGHTNINGD_POSTGRES_NO_VACUUM=1
 
+pip3 install --upgrade pip
 pip3 install --user poetry
 poetry config virtualenvs.create false --local
 poetry install
 
 git clone /~https://github.com/lightningnetwork/lightning-rfc.git ../lightning-rfc
 git submodule update --init --recursive
 
+# Used in docker image
+cd /work || return
+ls -la
+
 ./configure CC="$CC"
-cat config.vars
+# cat config.vars
 
 cat << EOF > pytest.ini
 [pytest]