Pin nightly rust for more controlled updating

ci/docker-rust-nightly/Dockerfile

@@ -1,8 +1,9 @@
 FROM solanalabs/rust
+ARG nightly=nightly-2018-08-14
 
 RUN set -x && \
-    rustup install nightly && \
-    rustup default nightly && \
-    rustup component add clippy-preview --toolchain=nightly && \
+    rustup install $nightly && \
+    rustup default $nightly && \
+    rustup component add clippy-preview --toolchain=$nightly && \
     rustc --version && \
     cargo --version

ci/docker-rust-nightly/README.md

@@ -1,6 +1,21 @@
 Docker image containing rust nightly and some preinstalled crates used in CI.
 
-This image may be manually updated by running `./build.sh` if you are a member
+This image may be manually updated by running `CI=true ./build.sh` if you are a member
 of the [Solana Labs](https://hub.docker.com/u/solanalabs/) Docker Hub
 organization, but it is also automatically updated periodically by
 [this automation](https://buildkite.com/solana-labs/solana-ci-docker-rust-nightly).
+
+## Moving to a newer nightly
+
+We pin the version of nightly (see the `ARG nightly=xyz` line in `Dockerfile)
+to avoid the build breaking at unexpected times, as occasionally nightly will
+introduce breaking changes.
+
+To update the pinned version:
+1. Locally update the `ARG nightly=xyz` line in `ci/docker-rust-nightly/Dockerfile`
+1. Run `ci/docker-rust-nightly/build.sh` to rebuild the nightly image locally
+1. Run `ci/docker-run.sh --nopull solanalabs/rust-nightly ci/test-nightly.sh` to
+   confirm the new nightly image builds.  Fix any issues as needed
+1. Run `CI=true ci/docker-rust-nightly/build.sh` to push the new nightly image to dockerhub.com.
+   **Note that all CI may be broken now until the next step is completed.**
+1. Merge the update to `ARG nightly=xyz` and any codebase adjustments needed.

ci/docker-rust-nightly/build.sh

@@ -3,4 +3,10 @@
 cd "$(dirname "$0")"
 
 docker build -t solanalabs/rust-nightly .
-docker push solanalabs/rust-nightly
+
+maybeEcho=
+if [[ -z $CI ]]; then
+  echo "Not CI, skipping |docker push|"
+  maybeEcho="echo"
+fi
+$maybeEcho docker push solanalabs/rust-nightly