feat: Simplify Dockerfile

Simplify Dockerfile by re-using toolchain and build dependencies from the image used to run iox's CI
system.

Dockerfile

@@ -1,34 +1,21 @@
 #syntax=docker/dockerfile:1.2
-FROM rust:slim-buster AS build
+FROM quay.io/influxdb/rust:ci as build
 
-# Build flatbuffers, a dependency of influxdb_iox
-ARG flatbuffers_version="v1.12.0"
-RUN apt-get update \
-  && apt-get install -y \
-    git make clang cmake llvm libssl-dev pkg-config \
-    --no-install-recommends \
-  && git clone -b ${flatbuffers_version} -- https://github.com/google/flatbuffers.git /usr/local/src/flatbuffers \
-  && cmake -S /usr/local/src/flatbuffers -B /usr/local/src/flatbuffers \
-    -G "Unix Makefiles" \
-    -DCMAKE_BUILD_TYPE=Release \
-  && make -C /usr/local/src/flatbuffers -j $(nproc) flatc \
-  && ln /usr/local/src/flatbuffers/flatc /usr/bin/flatc
+# cache mounts below may already exist and owned by root
+USER root
 
 # Build influxdb_iox
 COPY . /influxdb_iox
 WORKDIR /influxdb_iox
+
 RUN \
-  --mount=type=cache,id=influxdb_iox_rustup,sharing=locked,target=/usr/local/rustup \
-  rustup component add rustfmt
-RUN \
-  --mount=type=cache,id=influxdb_iox_rustup,sharing=locked,target=/usr/local/rustup \
   --mount=type=cache,id=influxdb_iox_registry,sharing=locked,target=/usr/local/cargo/registry \
   --mount=type=cache,id=influxdb_iox_git,sharing=locked,target=/usr/local/cargo/git \
   --mount=type=cache,id=influxdb_iox_target,sharing=locked,target=/influxdb_iox/target \
-  du -cshx /usr/local/rustup /usr/local/cargo/registry /usr/local/cargo/git /influxdb_iox/target && \
+  du -cshx /usr/local/cargo/registry /usr/local/cargo/git /influxdb_iox/target && \
   cargo build --target-dir /influxdb_iox/target --release && \
   cp /influxdb_iox/target/release/influxdb_iox /root/influxdb_iox && \
-  du -cshx /usr/local/rustup /usr/local/cargo/registry /usr/local/cargo/git /influxdb_iox/target
+  du -cshx /usr/local/cargo/registry /usr/local/cargo/git /influxdb_iox/target
 
 FROM debian:buster-slim