Pass --ignore-installed to pip in most of the cases.

The only case where it's fine to overwrite existing Python packages with
pip versions is when Platypush is running in a virtual environment.

Otherwise, keep the system-installed version if it's available, unless
its version is explicitly incompatible with the one reported in
`requirements.txt`.

platypush/install/docker/alpine.Dockerfile

@@ -18,7 +18,7 @@ RUN --mount=type=bind,source=.,target=/curdir \
 
 RUN /install/platypush/install/scripts/alpine/install.sh && \
     cd /install && \
-    pip install -U --no-input --no-cache-dir --no-deps . croniter --break-system-packages && \
+    pip install -U --no-input --no-cache-dir --no-deps --ignore-installed --break-system-packages . croniter && \
     rm -rf /install && \
     rm -rf /root/.cache && \
     apk del gcc git && \

platypush/install/docker/debian.Dockerfile

@@ -21,7 +21,7 @@ RUN --mount=type=bind,source=.,target=/curdir \
 
 RUN /install/platypush/install/scripts/debian/install.sh && \
     cd /install && \
-    pip install -U --no-input --no-cache-dir --no-deps . --break-system-packages && \
+    pip install -U --no-input --no-cache-dir --no-deps --ignore-installed --break-system-packages . && \
     rm -rf /install && \
     rm -rf /root/.cache && \
     apt remove -y git build-essential && \

platypush/install/docker/fedora.Dockerfile

@@ -19,7 +19,7 @@ RUN --mount=type=bind,source=.,target=/curdir \
 
 RUN /install/platypush/install/scripts/fedora/install.sh && \
   cd /install && \
-  pip install -U --no-input --no-cache-dir --no-deps . --break-system-packages && \
+  pip install -U --no-input --no-cache-dir --no-deps --ignore-installed --break-system-packages . && \
   rm -rf /install && \
   rm -rf /root/.cache && \
   dnf remove -y build-essential git && \

platypush/install/docker/ubuntu.Dockerfile

@@ -21,7 +21,7 @@ RUN --mount=type=bind,source=.,target=/curdir \
 
 RUN /install/platypush/install/scripts/debian/install.sh && \
     cd /install && \
-    pip install -U --no-input --no-cache-dir --no-deps . --break-system-packages && \
+    pip install -U --no-input --no-cache-dir --no-deps --ignore-installed --break-system-packages . && \
     rm -rf /install && \
     rm -rf /root/.cache && \
     apt remove -y git \

platypush/utils/manifest.py

@@ -475,6 +475,7 @@ class Dependencies:
                 yield (
                     'pip install -U --no-input '
                     + ('--no-cache-dir ' if self._is_docker else '')
+                    + ('--ignore-installed ' if not self._is_venv else '')
                     + (
                         '--break-system-packages '
                         if wants_break_system_packages