Fix various Dockerfile errors (#67)

Reviewed-on: #67

Dockerfile

@@ -7,7 +7,7 @@
 # For a containerized dev environment, see Dev Containers: https://guides.rubyonrails.org/getting_started_with_devcontainer.html
 
 # Make sure RUBY_VERSION matches the Ruby version in .ruby-version
-ARG RUBY_VERSION=3.4.1
+ARG RUBY_VERSION
 FROM docker.io/library/ruby:$RUBY_VERSION-slim AS base
 
 # Rails app lives here
@@ -29,11 +29,11 @@ FROM base AS build
 
 # Install packages needed to build gems
 RUN apt-get update -qq && \
-    apt-get install --no-install-recommends -y build-essential git pkg-config && \
+    apt-get install --no-install-recommends -y build-essential git pkg-config libyaml-dev && \
     rm -rf /var/lib/apt/lists /var/cache/apt/archives
 
 # Install application gems
-COPY Gemfile Gemfile.lock ./
+COPY Gemfile Gemfile.lock .ruby-version ./
 RUN bundle install && \
     rm -rf ~/.bundle/ "${BUNDLE_PATH}"/ruby/*/cache "${BUNDLE_PATH}"/ruby/*/bundler/gems/*/.git && \
     bundle exec bootsnap precompile --gemfile