util/docker: Replace use of sed with build args

Change-Id: I9ab101e06ed670dfe6802f9bd0df128d056446db
Signed-off-by: Tim Crawford <tcrawford@system76.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/77540
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Reviewed-by: Martin L Roth <gaumless@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

util/docker/coreboot-jenkins-test/Dockerfile

@@ -1,4 +1,6 @@
-FROM coreboot/coreboot-jenkins-node:{{SDK_VERSION}}
+ARG SDK_VERSION
+
+FROM coreboot/coreboot-jenkins-node:${SDK_VERSION}
 
 # Test the built image
 RUN mkdir -p /tmp/work && \