docker: Do not use pip Python package manager

Alpine Linux gets this with 3.19:

This is already installed with `pytest` via apk package manager.

```
15 78.20 error: externally-managed-environment
15 78.20
15 78.20 × This environment is externally managed
15 78.20 ╰─>
15 78.20     The system-wide python installation should be maintained using the system
15 78.20     package manager (apk) only.
15 78.20
15 78.20     If the package in question is not packaged already (and hence installable via
15 78.20     "apk add py3-somepackage"), please consider installing it inside a virtual
15 78.20     environment, e.g.:
15 78.20
15 78.20     python3 -m venv /path/to/venv
15 78.20     . /path/to/venv/bin/activate
15 78.20     pip install mypackage
15 78.20
15 78.20     To exit the virtual environment, run:
15 78.20
15 78.20     deactivate
15 78.20
15 78.20     The virtual environment is not deleted, and can be re-entered by re-sourcing
15 78.20     the activate file.
15 78.20
15 78.20     To automatically manage virtual environments, consider using pipx (from the
15 78.20     pipx package).
15 78.20
15 78.20 note: If you believe this is a mistake, please contact your Python installation or OS distribution provider. You can override this, at the risk of breaking your Python installation or OS, by passing --break-system-packages.
```

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>

docker/alpine/Dockerfile

@@ -33,8 +33,7 @@ RUN source /src/alpine/APKBUILD.in \
 	&& apk add \
 		--no-cache \
 		--update-cache \
-		$makedepends \
-	&& pip install pytest
+		$makedepends
 COPY . /src
 ARG PKGVER
 RUN cd /src \