Put env directly when building docker images

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

docker/production/Dockerfile

@@ -14,15 +14,13 @@ RUN yarn install --network-timeout 100000 \
 # Then, build the application binary
 FROM elixir:1.13-alpine AS builder
 
-ENV HEX_MIRROR="https://cdn.jsdelivr.net/hex"
-
 RUN apk add --no-cache build-base git cmake
 
 COPY mix.exs mix.lock ./
 ENV MIX_ENV=prod
 RUN mix local.hex --force \
     && mix local.rebar --force \
-    && mix deps.get
+    && HEX_MIRROR="https://cdn.jsdelivr.net/hex" mix deps.get
 
 COPY lib ./lib
 COPY priv ./priv