Install curl in the Dockerfile for healthchecks (#209)

### What

If we have `curl`, we can do a very basic healthcheck like `curl
localhost:8100/health`. Note that it must be installed _after_ the
second `FROM`.

### How

<!-- How is it trying to accomplish it (what are the implementation
steps)? -->

Co-authored-by: Tom Harding <[email protected]>

.dockerignore

@@ -0,0 +1 @@
+target

ConfigServer.Dockerfile

@@ -19,5 +19,8 @@ RUN cargo build --release --all-targets
 ## Copy the binaries and serve the configuration server
 FROM debian:buster-slim as ndc-postgres
 ENV PORT=9100
+
+## Install curl for the health check
+RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install --no-install-recommends --assume-yes curl
 COPY --from=build /app/target/release/ndc-postgres ./ndc-postgres
 CMD ["sh", "-c", "./ndc-postgres configuration serve"]