Add ARG RUSTFLAGS into Dockerfile (#4556)

* Add ARG RUSTFLAGS into Dockerfile 

Allows passing RUSTFLAGS through --build-arg

* Remove invalid ARG syntax

docker/Dockerfile

@@ -19,6 +19,7 @@ FROM --platform=${BUILDPLATFORM} ${AMD_BUILDER_IMAGE} AS build-amd64
 
 ARG CARGO_BUILD_FEATURES
 ARG RUST_RELEASE_MODE
+ARG RUSTFLAGS
 
 WORKDIR /lemmy
 
@@ -48,6 +49,7 @@ FROM --platform=linux/amd64 ${ARM_BUILDER_IMAGE} AS build-arm64
 
 ARG RUST_RELEASE_MODE
 ARG CARGO_BUILD_FEATURES
+ARG RUSTFLAGS
 
 WORKDIR /home/lemmy/src
 USER 10001:10001