feat: Test building with swift:slim, as a matrix strategy in ci.

Dockerfile

@@ -1,4 +1,6 @@
-FROM docker.io/swift:latest AS build
+ARG SWIFT_VERSION="swift:latest"
+
+FROM docker.io/${SWIFT_VERSION} AS build
 
 RUN export DEBIAN_FRONTEND=noninteractive DEBCONF_NONINTERACTIVE_SEEN=true \
     && apt-get -q update \
@@ -13,7 +15,7 @@ RUN rustup default stable && \
 	/root/.cargo/bin/bob install nightly && \
 	/root/.cargo/bin/bob use nightly
 
-FROM docker.io/swift:latest
+FROM docker.io/${SWIFT_VERSION}
 
 RUN export DEBIAN_FRONTEND=noninteractive DEBCONF_NONINTERACTIVE_SEEN=true \
     && apt-get -q update \