Use ruby-slim-stretch as base instead of rvm

Dockerfile.diaspora

@@ -1,4 +1,4 @@
-FROM debian:stretch as base
+FROM ruby:2.4.4-slim-stretch
 
 ARG DEBIAN_FRONTEND=noninteractive
 RUN apt-get update \
@@ -16,20 +16,6 @@ RUN apt-get update \
         libpq-dev \
         cmake \
         nodejs \
-        # Packages for RVM
-        gawk \
-        libyaml-dev \
-        libsqlite3-dev \
-        sqlite3 \
-        autoconf \
-        libgmp-dev \
-        libgdbm-dev \
-        libncurses5-dev \
-        automake \
-        bison \
-        libffi-dev \
-        libgmp-dev \
-        libreadline-dev \
     && rm -rf /var/lib/apt/lists/*
 
 ARG EXT_UID
@@ -41,24 +27,6 @@ USER diaspora
 RUN mkdir /home/diaspora/diaspora
 WORKDIR /home/diaspora/diaspora
 
-# Install RVM + Ruby
-ENV LANG=C.UTF8
-
-RUN curl -sSL https://rvm.io/mpapis.asc | gpg --import - \
-    && curl -L https://s.diaspora.software/1t | bash \
-    && rm -rf ~/.gnupg
-SHELL ["/bin/bash", "--login", "-c"]
-
-RUN rvm install 2.4
-
-# Install bundler
-RUN echo diaspora > .ruby-gemset \
-    && echo 2.4 > .ruby-version \
-    && touch Gemfile \
-    && cd ../diaspora \
-    && gem install bundler \
-    && rm .ruby-* Gemfile
-
 # Finish setup
 COPY ./start.sh ../start.sh