Default back to wasm-5.8.0-RELEASE at this moment for docker image

Even with 5.8 being the default, users can still use 5.9 by specifying
the toolchain version in the .swift-version file.

Dockerfile

@@ -1,4 +1,4 @@
-FROM ghcr.io/swiftwasm/swift:5.9-focal
+FROM ghcr.io/swiftwasm/swift:5.8-focal
 
 LABEL maintainer="SwiftWasm Maintainers <hello@swiftwasm.org>"
 LABEL Description="Carton is a watcher, bundler, and test runner for your SwiftWasm apps"
@@ -16,7 +16,7 @@ RUN export DEBIAN_FRONTEND=noninteractive DEBCONF_NONINTERACTIVE_SEEN=true && ap
   rm -r /var/lib/apt/lists/*
 
 ENV CARTON_ROOT=/root/.carton
-ENV CARTON_DEFAULT_TOOLCHAIN=wasm-5.9-SNAPSHOT-2023-08-01-a
+ENV CARTON_DEFAULT_TOOLCHAIN=wasm-5.8.0-RELEASE
 
 RUN mkdir -p $CARTON_ROOT/sdk && \
   mkdir -p $CARTON_ROOT/sdk/$CARTON_DEFAULT_TOOLCHAIN && \

Sources/CartonHelpers/DefaultToolchain.swift

@@ -13,4 +13,4 @@
 // limitations under the License.
 
 // TODO: make this `public let defaultToolchainVersion = "wasm-5.9.0-RELEASE"`
-public let defaultToolchainVersion = "wasm-5.9-SNAPSHOT-2023-08-01-a"
+public let defaultToolchainVersion = "wasm-5.9-SNAPSHOT-2023-12-12-a"