[ruby] Build with base ruby 3.0 to bootstrap from git sources

We recently updated the minimum required base ruby version to 3.0
(ruby/ruby#9976). This change updates the
Dockerfile to build base ruby from tarball and install it instead of
using Ubuntu focal's ruby package, which is still 2.7.

projects/ruby/Dockerfile

@@ -19,11 +19,13 @@ FROM gcr.io/oss-fuzz-base/base-builder
 RUN apt-get update -y && \
  apt-get install -y libssl-dev libyaml-dev libreadline6-dev \
  zlib1g-dev libncurses5-dev libffi-dev \
- bison autoconf ruby pkg-config
+ bison autoconf pkg-config
+RUN mkdir $SRC/fuzz
+COPY build_baseruby.sh $SRC/fuzz/
+RUN $SRC/fuzz/build_baseruby.sh
 
 RUN git clone https://github.com/ruby/ruby.git
 WORKDIR ruby
 
 COPY build.sh $SRC/
-RUN mkdir $SRC/fuzz
 COPY *.rb *.c *.options $SRC/fuzz/

projects/ruby/build_baseruby.sh

@@ -0,0 +1,38 @@
+#!/bin/bash
+# Copyright 2024 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+################################################################################
+
+# This script builds and installs the base ruby, which is used to bootstrap ruby from git sources.
+
+set -euxo pipefail
+
+# NOTE: You can find tarball URLs and SHA256 checksums at https://www.ruby-lang.org/en/downloads
+ruby_tarball_url=https://cache.ruby-lang.org/pub/ruby/3.0/ruby-3.0.6.tar.gz
+ruby_tarball_sha256=6e6cbd490030d7910c0ff20edefab4294dfcd1046f0f8f47f78b597987ac683e
+
+# Download and verify the tarball
+build_dir="$(mktemp -d -t build_baseruby.XXXXXX)"
+curl -L -o "$build_dir/ruby.tar.gz" "$ruby_tarball_url"
+echo "$ruby_tarball_sha256 $build_dir/ruby.tar.gz" | sha256sum -c -
+
+# Extract the tarball
+tar -C "$build_dir" -xzf "$build_dir/ruby.tar.gz" --strip-components=1
+
+# Build and install the base ruby
+cd "$build_dir"
+./configure --disable-install-doc
+make -j"$(nproc)"
+make install