Dockerfile: update image to "main"

This is compatible with the latest main, since 45 has not been
released yet.

.github/Dockerfile

@@ -1,6 +1,6 @@
 FROM ghcr.io/distroless/alpine-base:latest
 
-ENV BRANCH=gnome-43
+ENV BRANCH=main
 # distroless/alpine-base only has main by default
 RUN echo "" >> /etc/apk/repositories \
     && echo https://dl-cdn.alpinelinux.org/alpine/edge/community >> /etc/apk/repositories