Update Dockerfile

src/Dockerfile

@@ -59,9 +59,9 @@ RUN \
 # For some reason the rustup script is unable to correctly identify the
 # environment if we are building an i386 image on an x86_64 system, so we need
 # to provide this information manually.
-        $(if [ "$TARGETPLATFORM" = "linux/386" ]; then echo "--default-host i686-unknown-linux-gnu"; fi); \
+        $(if [ "$TARGETPLATFORM" = "linux/386" ]; then echo "--default-host i686-unknown-linux-gnu"; fi) && \
+        . "$HOME/.cargo/env"; \
     fi && \
-    . "$HOME/.cargo/env" && \
 # Install certbot.
     pip3 install -r /requirements.txt && \
 # And the supported extra authenticators.