Fix alpine armv7 build

Reference: https://github.com/messense/rust-musl-cross/pull/34

docker/armv7/Dockerfile.alpine

@@ -38,6 +38,7 @@ RUN rustup set profile minimal
 
 ENV USER "root"
 ENV RUSTFLAGS='-C link-arg=-s'
+ENV CFLAGS_armv7_unknown_linux_musleabihf="-mfpu=vfpv3-d16"
 
 # Creates a dummy project used to grab dependencies
 RUN USER=root cargo new --bin /app