docker: Simplify build/install; dedup cargo commands; enable gc.

Signed-off-by: Jason Volk <jason@zemos.net>

docker/Dockerfile.complement

@@ -8,7 +8,6 @@ WORKDIR /
 COPY --link --from=input . .
 
 RUN \
---mount=type=cache,dst=/var/lock/uwu/local,sharing=locked \
 --mount=type=cache,dst=${var_cache},sharing=locked \
 --mount=type=cache,dst=${var_lib_apt},sharing=locked \
 <<EOF
@@ -157,7 +156,6 @@ ARG complement_run=".*"
 
 WORKDIR /
 RUN \
---mount=type=cache,dst=/var/lock/uwu/local,sharing=locked \
 --mount=type=cache,dst=${var_cache},sharing=locked \
 --mount=type=cache,dst=${var_lib_apt},sharing=locked \
 --mount=type=cache,dst=/go/pkg/mod/cache,sharing=locked \