Re-implement Docker workflow using GitHub Actions (#326)

* Recreate a docker dir with all docker files inside and add init.sh
* Add docker/README.md with example docker-compose.xml
* Add mysql vars for mempool-config.json
* Port can be set from env var in docker-compose.xml
* Update docker-compose.xml example to use latest tag
* Remove armv7 architecture from build workflow
* Remove master git-hash file
* Remove useless ':' in sed

docker/backend/Dockerfile

@@ -0,0 +1,27 @@
+FROM node:12-buster-slim AS builder
+
+WORKDIR /build
+COPY . .
+
+RUN apt-get update
+RUN apt-get install -y build-essential python3 pkg-config
+RUN npm ci --production
+RUN npm i typescript
+RUN npm run build
+
+FROM node:12-buster-slim
+
+WORKDIR /backend
+
+COPY --from=builder /build/ .
+
+RUN chmod +x /backend/start.sh
+RUN chmod +x /backend/wait-for-it.sh
+
+RUN chown -R 1000:1000 /backend && chmod -R 755 /backend
+
+USER 1000
+
+EXPOSE 8999
+
+CMD ["/backend/start.sh"]