Fix building mimalloc on armv6

The armv6 builds need a specific location for the libatomic.a file.
This commit fixes that by adding a RUSTFLAGS argument for this.

Also removed the `link-arg=-s` since this is now already done during via the release profile
And removed the CFLAGS for armv7, this is already fixed by default in the blackdex/rust-musl images.

docker/armv7/Dockerfile.alpine

@@ -44,8 +44,6 @@ ENV DEBIAN_FRONTEND=noninteractive \
 RUN mkdir -pv "${CARGO_HOME}" \
     && rustup set profile minimal
 
-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
@@ -59,7 +57,8 @@ COPY ./build.rs ./build.rs
 RUN rustup target add armv7-unknown-linux-musleabihf
 
 # Configure the DB ARG as late as possible to not invalidate the cached layers above
-ARG DB=sqlite,mysql,postgresql
+# Enable MiMalloc to improve performance on Alpine builds
+ARG DB=sqlite,mysql,postgresql,enable_mimalloc
 
 # Builds your dependencies and removes the
 # dummy project, except the target folder