Remove the gratuitous stage `base` in the Rust Dockerfile. It's not
necessary, and it's causing confusion.
-FROM rust:1-slim-buster AS base
+FROM rust:1-slim-buster AS build
ARG CARGO_PROFILE=debug
@@ -34,14 +34,6 @@ COPY rust rust
WORKDIR /grapl/rust
-
-# build
-#
-# the `base` stage isn't used by the build system, but let's
-# make `base` and `build` in case it becomes convenient.
-################################################################################
-FROM base AS build
-
RUN --mount=type=cache,mode=0777,target=/root/.cache/sccache \
--mount=type=secret,id=rust_env,dst=/grapl/env \
source /grapl/env; \
