ci: Use the new redoxer docker image for redox builds

.gitlab-ci.yml

-image: 'rust:latest'
+image: "redoxos/redoxer"
 
 variables:
     CARGO_HOME: $CI_PROJECT_DIR/cargo
@@ -31,27 +31,8 @@ linux:stable:
     - TOOLCHAIN= make tests
 
 redox:
-  variables:
-    CC: "x86_64-unknown-redox-gcc"
-  allow_failure: true
-  before_script:
-    - apt-get update -qq
-    - apt-get install -qq tar
-    - '[ -e "prefix" ] || (mkdir prefix && wget -O - https://static.redox-os.org/toolchain/x86_64-unknown-redox/gcc-install.tar.gz |
-          tar --extract --gzip --directory prefix)'
-    - export PATH="$PWD/prefix/bin:$PATH"
-    - export RUSTUP_TOOLCHAIN="$PWD/prefix"
-
-        # Prepare xargo
-    - cargo install xargo
-    - '[ -e "rust" ] || git clone -b redox-2019-04-06 "https://gitlab.redox-os.org/redox-os/rust" --recursive'
-    - export CARGO=xargo
-    - export XARGO_HOME="$PWD/build/xargo"
-    - export XARGO_RUST_SRC="$PWD/rust/src"
-    - rustup default nightly-2019-04-06
-    - rustup target add x86_64-unknown-redox
   script:
-    - make TOOLCHAIN= REDOX=1
+    - redoxer build
 
 pages:
   image: hrektts/mdbook