Document how to build rustc under the same conditions as in CI

Sometimes the CI build fails, and when one tries running it locally, it succeeds.

@mati865 mentions here that one can build locally on linux under the same conditions as CI by doing:

DEPLOY=1 ./src/ci/docker/run.sh dist-x86_64-linux.

This other user had to use this instead:

#!/bin/bash

# Clean up from previous build
rm -rf build

# Configuration which is always enabled
export RUST_RELEASE_CHANNEL=nightly
RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --enable-sccache"
RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --disable-manage-submodules"
RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --enable-locked-deps"
RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --enable-cargo-native-static"
RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --set rust.codegen-units-std=1"

# Deploy configuration
RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --release-channel=$RUST_RELEASE_CHANNEL"
RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --enable-llvm-static-stdcpp"
RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --set rust.remap-debuginfo"
RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --debuginfo-level-std=1"

RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --enable-llvm-assertions"
RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --set rust.verify-llvm-ir"

RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --enable-debug-assertions"
RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --set rust.jemalloc"

# Configure and make
./configure $RUST_CONFIGURE_ARGS
make -j $(nproc)

we should document exactly how to try to reproduce a CI failure locally inside docker, and then how to try to reproduce it outside docker but still using all other options that are set in CI but not by default (like using jemalloc instead of the system allocator).

This information should be complemented with a fix to #443 that explains how to attach a debugger on these scenarios.

Start with src/ci/docker/run.sh and the documented ./configure and make commands in the issue, then compare them with the CI configuration to identify options not enabled by default. Done means documenting both Docker-based reproduction and an outside-Docker reproduction using the relevant CI options, with debugger guidance coordinated with #443.