Dockerfile: build docker image "from scratch" instad of "from rust"

RUSTC_BOOTSTRAP=1 doesn't work so well anymore in stable, so we had to
migrate to nightly - which required us to install the toolchain in an
image that already has one. If we install rust manually in a
debian-slim, we save ~500MB.

We are also pinning nightly to a version where coverage works.
Ref: rust-lang/rust#93054

Dockerfile

@@ -2,8 +2,33 @@
 # This file is subject to the terms and conditions defined in
 # file 'LICENSE', which is part of this source code package.
 
-FROM rust:slim
-RUN set -e -x; \
+# Adapted from https://github.com/rust-lang/docker-rust/blob/master/Dockerfile-slim.template
+
+FROM debian:bullseye-slim
+
+ENV RUSTUP_HOME=/usr/local/rustup \
+    CARGO_HOME=/usr/local/cargo \
+    PATH=/usr/local/cargo/bin:$PATH
+
+RUN set -eux; \
+    apt-get update; \
+    apt-get install -y --no-install-recommends \
+        ca-certificates \
+        gcc \
+        libc6-dev \
+        wget \
+        ; \
+    wget https://sh.rustup.rs -O rustup-init; \
+    bash rustup-init -y --no-modify-path --profile minimal --default-toolchain nightly-2022-01-14; \
+    rm rustup-init; \
+    chmod -R a+w $RUSTUP_HOME $CARGO_HOME; \
+    rustup --version; \
+    cargo --version; \
+    rustc --version; \
+    apt-get remove -y --auto-remove \
+        wget \
+        ; \
+    rm -rf /var/lib/apt/lists/*; \
     rustup component add llvm-tools-preview; \
     cargo install cargo-llvm-cov