Clarify the origin of rust-$TARGET CI Docker images

Author: rami3l

.github/workflows/linux-builds-on-master.yaml

@@ -96,7 +96,7 @@ jobs:
             *)                DOCKER="$TARGET" ;;
           esac
           echo "DOCKER=$DOCKER" >> $GITHUB_ENV
-      - name: Fetch the docker
+      - name: Fetch the base docker images from rust-lang/rust
         run: bash ci/fetch-rust-docker.bash "${TARGET}"
       - name: Maybe build a docker from there
         run: |

.github/workflows/linux-builds-on-pr.yaml

@@ -90,7 +90,7 @@ jobs:
             *)                DOCKER="$TARGET" ;;
           esac
           echo "DOCKER=$DOCKER" >> $GITHUB_ENV
-      - name: Fetch the docker
+      - name: Fetch the base docker images from rust-lang/rust
         run: bash ci/fetch-rust-docker.bash "${TARGET}"
       - name: Maybe build a docker from there
         run: |

.github/workflows/linux-builds-on-stable.yaml

@@ -120,7 +120,7 @@ jobs:
             *)                DOCKER="$TARGET" ;;
           esac
           echo "DOCKER=$DOCKER" >> $GITHUB_ENV
-      - name: Fetch the docker
+      - name: Fetch the base docker images from rust-lang/rust
         run: bash ci/fetch-rust-docker.bash "${TARGET}"
       - name: Maybe build a docker from there
         run: |

ci/actions-templates/linux-builds-template.yaml

@@ -129,7 +129,7 @@ jobs:
             *)                DOCKER="$TARGET" ;;
           esac
           echo "DOCKER=$DOCKER" >> $GITHUB_ENV
-      - name: Fetch the docker
+      - name: Fetch the base docker images from rust-lang/rust
         run: bash ci/fetch-rust-docker.bash "${TARGET}"
       - name: Maybe build a docker from there
         run: |

ci/fetch-rust-docker.bash

@@ -12,9 +12,14 @@ TARGET="$1"
 
 RUST_REPO="https://github.com/rust-lang/rust"
 ARTIFACTS_BASE_URL="https://ci-artifacts.rust-lang.org/rustc-builds"
-LOCAL_DOCKER_TAG="rust-$TARGET"
 
-# Use images from rustc master
+# A `Dockerfile` under `rustup`'s `ci/docker` directory may start with `FROM rust-$TARGET`.
+# This means it is using an S3-cached Docker image provided by `rustc`'s CI.
+LOCAL_DOCKER_TAG="rust-$TARGET"
+# The following is a mapping from `$TARGET`s to cached Docker images built from `Dockerfile`s under
+# <https://github.com/rust-lang/rust/blob/master/src/ci/docker/host-x86_64/>,
+# e.g. `FROM rust-aarch64-unknown-linux-musl` means the base `Dockerfile` to look at is located under
+# <https://github.com/rust-lang/rust/blob/master/src/ci/docker/host-x86_64/dist-arm-linux>.
 case "$TARGET" in
   aarch64-unknown-linux-gnu)       image=dist-aarch64-linux ;;
   aarch64-unknown-linux-musl)      image=dist-arm-linux ;;