Remove some duplicate code in Dockerfile with the help of some variables

docker/arm32v7/Dockerfile.alpine

@@ -32,7 +32,6 @@ RUN rustup set profile minimal
 ENV USER "root"
 ENV RUSTFLAGS='-C link-arg=-s'
 
-
 # Creates a dummy project used to grab dependencies
 RUN USER=root cargo new --bin /app
 WORKDIR /app