ci: alpine Docker image

This would be a much more lightweight image, with the same tool. Furthermore, it will be a lot faster and easier to add extra packages such as `curl` (i.e. a fast and clean `apk add --no-cache curl` instead of a slow `apt-get update && apt-get install curl -y` which adds harder to remove cache.
This does still need you to set up a pipeline to create `ghcr.io/tomwright/dasel:alpine`.

docker/alpine.Dockerfile

@@ -0,0 +1,10 @@
+FROM alpine
+
+ARG daselpath=./dasel
+
+WORKDIR /root
+COPY $daselpath /usr/local/bin/dasel
+RUN chmod +x /usr/local/bin/dasel
+
+ENTRYPOINT ["/usr/local/bin/dasel"]
+CMD []