Change docker image to use nightly-2021-02-20 tag

scripts/raspberry-cross-build.sh

@@ -12,5 +12,5 @@ docker run \
     -e WASM_BUILD_TOOLCHAIN=nightly-2021-02-20 \
     --volume ${WORKSPACE_ROOT}/:/home/cross/project \
     --volume ${HOME}/.cargo/registry:/home/cross/.cargo/registry \
-    joystream/rust-raspberry \
+    joystream/rust-raspberry:nightly-2021-02-20 \
     build --release -p joystream-node