perf: improve action download speed by using an alpine image

The action's image is now based on alpine, which reduced drastically it's size. The previous compressed size was ~352 MB, now it's only ~56 MB.

Resolves #37

Author: wagoid

Dockerfile

@@ -1,4 +1,6 @@
-FROM node:10
+FROM node:12-alpine3.9
+
+RUN apk --no-cache add git
 
 COPY package*.json /