fix joystream-node.Dockerfile

joystream-node.Dockerfile

@@ -5,9 +5,10 @@ COPY . /joystream
 
 # Build all cargo crates
 # Ensure our tests and linter pass before actual build
-RUN WASM_BUILD_TOOLCHAIN=nightly-2020-05-23 BUILD_DUMMY_WASM_BINARY=1 cargo clippy --release --all -- -D warnings \
-    WASM_BUILD_TOOLCHAIN=nightly-2020-05-23 cargo test --release --all && \
-    WASM_BUILD_TOOLCHAIN=nightly-2020-05-23 cargo build --release
+ENV WASM_BUILD_TOOLCHAIN=nightly-2020-05-23
+RUN BUILD_DUMMY_WASM_BINARY=1 cargo clippy --release --all -- -D warnings && \
+    cargo test --release --all && \
+    cargo build --release
 
 FROM debian:stretch
 LABEL description="Joystream node"