CI: automatic builds (#1)

Adds CI pipelines to build and push the project to the Gitea Docker repository.

Co-authored-by: Julien Valverdé <julien.valverde@mailo.com>
Reviewed-on: https://git.jvalver.de/Thilawyn/protonmail-bridge-docker/pulls/1

build/Dockerfile

@@ -1,5 +1,4 @@
-# Use carlosedp/golang for riscv64 support
-FROM carlosedp/golang:1.18 AS build
+FROM golang:1.20-bookworm AS build
 
 # Install dependencies
 RUN apt-get update && apt-get install -y git build-essential libsecret-1-dev