Remove redundant TARGETPLATFORM in Dockerfile

Setting platform to predefined $TARGETPLATFORM is
redundant as this is the default behavior.

Signed-off-by: Daniel Zullo <[email protected]>

Author: daniel-zullo-frequenz

.github/containers/test-installation/Dockerfile

@@ -3,7 +3,7 @@
 # This Dockerfile is used to test the installation of the python package in
 # multiple platforms in the CI. It is not used to build the package itself.
 
-FROM --platform=${TARGETPLATFORM} python:3.11-slim
+FROM python:3.11-slim
 
 RUN apt-get update -y && \
     apt-get install --no-install-recommends -y \