Dockerfile: update buildx to v0.20.0

Update the buildx cli plugin used in the dev-container

full diff: https://github.com/docker/buildx/compare/v0.18.0..v0.20.0

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Dockerfile

@@ -8,7 +8,7 @@ ARG GO_VERSION=1.23.4
 ARG XX_VERSION=1.6.1
 ARG GOVERSIONINFO_VERSION=v1.4.1
 ARG GOTESTSUM_VERSION=v1.10.0
-ARG BUILDX_VERSION=0.18.0
+ARG BUILDX_VERSION=0.20.0
 ARG COMPOSE_VERSION=v2.30.3
 
 FROM --platform=$BUILDPLATFORM tonistiigi/xx:${XX_VERSION} AS xx