Dockerfile Nightly Version of Rust Doesn't Have Clippy

[YazzyYaz]

When running the Dockerfile manually, I run into the following error on this line:

Step 8/13 : RUN rustup override set nightly-2019-07-04
 ---> Running in 8d8f71b6b684
info: syncing channel updates for 'nightly-2019-07-04-x86_64-unknown-linux-gnu'
info: latest update on 2019-07-04, rust version 1.37.0-nightly (088b98730 2019-07-03)
error: component 'clippy' for target 'x86_64-unknown-linux-gnu' is unavailable for download for channel nightly-2019-07-04
The command '/bin/sh -c rustup override set nightly-2019-07-04' returned a non-zero code: 1

My fix for it was to update the Nightly Date to:

RUN rustup override set nightly-2019-11-04

It installs clippy now. For reference, following link shows missing packages from nightly installs:
https://rust-lang.github.io/rustup-components-history/index.html

I can submit a PR to your Dockerfile, but wanted to share this first with you.

[lacabra]

Thanks @YazzyYaz. I will let @fredfortier handle the change. For now point out that this variable should be in sync with what the rest of the core uses here