Fix container build

There was an error in building container immage as python expect to use
venv instead of installing p3exporter to system python. As this
container is only for illustrating how it can work we decided to simply
add optione `--break-system-packages` to pip command to override the
default behavior.

We also moved the container manifest to the more general `Containerfile`.

Author: cmeissner

Dockerfile renamed to Containerfile

@@ -7,7 +7,7 @@ WORKDIR /usr/src/app
 COPY . /usr/src/app/
 RUN apk update && \
     apk add python3 py3-pip build-base python3-dev linux-headers && \
-    pip3 install -e . && \
+    pip3 install --break-system-packages -e . && \
     apk del py3-pip build-base python3-dev linux-headers
 
 EXPOSE 5876