Upgrade dependencies; Node.js to v20 in client build

I rolled back to v18 a while ago due to some Docker for Mac issues that
should be resolved now. With v21 out this week, I think it makes sense
to try the latest available LTS again.

Dockerfile

@@ -4,7 +4,7 @@
 ARG ALPINE_BASE=docker.io/library/alpine:3.18
 ARG GOLANG_BASE=docker.io/library/golang:1.21-alpine3.18
 # The Node.js image does not require any particular OS.
-ARG NODEJS_BASE=docker.io/library/node:18-alpine
+ARG NODEJS_BASE=docker.io/library/node:20-alpine
 # See https://gstreamer.freedesktop.org/download/.
 ARG GSTREAMER_VERSION=1.22.6