Fix docker build (#601)

* Fix Docker build

I didn't test it locally, but theratically should be able to fix the
[docker build failure](https://github.com/chainx-org/ChainX/runs/4324208279?check_suite_focus=true)
as long as we use the same rustc version to the one used in GitHub Action.

* Add a note in ci.yml

.github/workflows/ci.yaml

@@ -17,6 +17,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest]
+        # NOTE: Do not forget to also update the toolchain in Dockerfile when updating the toolchain here.
         toolchain: [nightly-2021-06-29]
 
     steps:

Dockerfile

@@ -4,7 +4,7 @@ LABEL description="The build stage for ChainX. We create the ChainX binary in th
 
 ARG PROFILE=release
 ARG APP=chainx
-ARG RUSTC_VERSION=nightly-2021-03-01
+ARG RUSTC_VERSION=nightly-2021-06-29
 
 WORKDIR /$APP