Pin Docker image debian:buster-slim

debian:buster-slim
->
debian:buster-slim@sha256:557ee531b81ce380d012d83b7bb56211572e5d6088d3e21a3caef7d7ed7f718b

 [atomist:generated]
 [atomist-skill:atomist/docker-base-image-policy]

Signed-off-by: Atomist Bot <bot@atomist.com>

react-rust-postgres/backend/Dockerfile

@@ -38,7 +38,7 @@ FROM base AS builder
 
 RUN cargo build --release --offline
 
-FROM debian:buster-slim
+FROM debian:buster-slim@sha256:557ee531b81ce380d012d83b7bb56211572e5d6088d3e21a3caef7d7ed7f718b
 
 ENV ROCKET_ENV=production