Directly pass in RUBY_TARGET to Rake task

Previously rb-sys-dock relied on a static `RUBY_TARGET` being set, but
this value needs to be overriden in case an alias of
`x86_64-linux-gnu` is used for `x86_64-linux`.

docker/setup/rubybashrc.sh

@@ -52,6 +52,7 @@ main() {
   echo "export PATH=\"/usr/local/cargo/bin:\$PATH\"" >> "$OUTFILE"
   echo "export RUSTUP_HOME=\"$RUSTUP_HOME\"" >> "$OUTFILE"
   echo "export CARGO_HOME=\"$CARGO_HOME\"" >> "$OUTFILE"
+  echo "export RUBY_TARGET=\"$RUBY_TARGET\"" >> "$OUTFILE"
   echo "export RCD_PLATFORM=\"$RUBY_TARGET\"" >> "$OUTFILE"
   echo "export RUST_TARGET=\"$RUST_TARGET\"" >> "$OUTFILE"
   echo "export RUSTUP_DEFAULT_TOOLCHAIN=\"$RUSTUP_DEFAULT_TOOLCHAIN\"" >> "$OUTFILE"

gem/exe/rb-sys-dock

@@ -290,6 +290,10 @@ def tmp_target_dir
   @tmp_target_dir = dir
 end
 
+def ruby_target
+  OPTIONS.fetch(:platform)
+end
+
 def working_directory
   OPTIONS.fetch(:directory)
 end
@@ -310,8 +314,8 @@ def default_command_to_run(input_args)
   input_cmd = input_args.empty? ? "true" : input_args.join(" ")
 
   if OPTIONS[:build]
-    with_bundle = +"test -f Gemfile && bundle install && #{input_cmd} && bundle exec rake native:$RUBY_TARGET gem"
-    without_bundle = "#{input_cmd} && rake native:$RUBY_TARGET gem"
+    with_bundle = +"test -f Gemfile && bundle install && #{input_cmd} && bundle exec rake native:#{ruby_target} gem RUBY_TARGET=#{ruby_target}"
+    without_bundle = "#{input_cmd} && rake native:#{ruby_target} gem RUBY_TARGET=#{ruby_target}"
     logger.info("Running default build command (rake native:#{ruby_platform} gem)")
     "bash -c '(#{with_bundle}) || (#{without_bundle})'"
   else
@@ -391,7 +395,6 @@ def rcd(input_args)
       -e RB_SYS_DOCK_TMPDIR="/tmp/rb-sys-dock" \
       -e RB_SYS_CARGO_TARGET_DIR=#{tmp_target_dir.inspect} \
       -e RUBY_CC_VERSION="#{ruby_versions}" \
-      -e RUBY_TARGET="#{OPTIONS.fetch(:platform)}" \
       -e RAKEOPT \
       -e TERM \
       -w #{working_directory} \