ci: install missing dependency to build image (#391)

Author: Revolyssup

dockerfiles/Dockerfile.fpm

@@ -3,6 +3,7 @@ FROM ubuntu:focal
 RUN DEBIAN_FRONTEND=noninteractive apt-get update \
     && DEBIAN_FRONTEND=noninteractive apt-get install -y git \
     && DEBIAN_FRONTEND=noninteractive apt-get install -y ruby ruby-dev rubygems build-essential rpm \
+    && gem install dotenv -v 2.8.1 \
     && gem install fpm \
     && fpm --version