diff --git a/lib/runtime.func b/lib/runtime.func
index f4030dd..8b232fb 100644
--- a/lib/runtime.func
+++ b/lib/runtime.func
@@ -2432,12 +2432,16 @@ EOF
       }
       msg_ok "Updated ruby-build definitions"
     fi
-    $STD "$RBENV_BIN" install "$RUBY_VERSION" || {
-      msg_error "Failed to install Ruby $RUBY_VERSION"
+    # /tmp is tmpfs on Debian 13 and counts against the container's memory
+    local build_tmp
+    build_tmp="$(mktemp -d -p /var/tmp ruby-build.XXXXXX 2>/dev/null)" || build_tmp=""
+    $STD env TMPDIR="${build_tmp:-/tmp}" RUBY_CONFIGURE_OPTS="--disable-install-doc ${RUBY_CONFIGURE_OPTS:-}" "$RBENV_BIN" install "$RUBY_VERSION" || {
+      msg_error "Failed to install Ruby $RUBY_VERSION - the build log is in ${build_tmp:-/tmp}"
       cd "$PREV_DIR" 2>/dev/null || cd /
-    rm -rf "$TMP_DIR"
+      rm -rf "$TMP_DIR"
       return 150
     }
+    [[ -n "$build_tmp" ]] && rm -rf "$build_tmp"
   fi
 
   "$RBENV_BIN" global "$RUBY_VERSION" || {
diff --git a/tests/ruby-build-env.sh b/tests/ruby-build-env.sh
new file mode 100644
index 0000000..e5d18ad
--- /dev/null
+++ b/tests/ruby-build-env.sh
@@ -0,0 +1,74 @@
+#!/usr/bin/env bash
+set -uo pipefail
+
+CORE_DIR="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")/.." && pwd)"
+eval "$(awk '/^setup_ruby\(\) \{/,/^}/' "$CORE_DIR/lib/runtime.func")"
+
+TEST_DIR="$(mktemp -d)"
+trap 'rm -rf -- "$TEST_DIR"' EXIT
+passed=0 failed=0
+
+check() {
+  if [[ "$3" == "$2" ]]; then
+    echo "PASS $1"
+    passed=$((passed + 1))
+  else
+    echo "FAIL $1: expected <$2>, got <$3>" >&2
+    failed=$((failed + 1))
+  fi
+}
+
+# shellcheck disable=SC2034  # read by setup_ruby
+STD=""
+msg_info() { :; }
+msg_ok() { :; }
+msg_warn() { :; }
+msg_error() { :; }
+ensure_apt_working() { :; }
+apt-cache() { echo found; }
+apt() { :; }
+cache_installed_version() { :; }
+
+export HOME="$TEST_DIR/home"
+RBENV="$HOME/.rbenv"
+mkdir -p "$RBENV/bin" "$RBENV/plugins/ruby-build/share/ruby-build"
+: >"$RBENV/plugins/ruby-build/share/ruby-build/3.4.7"
+cat >"$RBENV/bin/rbenv" <<EOF
+#!/usr/bin/env bash
+case "\$1" in
+install)
+  {
+    echo "tmpdir=\$TMPDIR"
+    echo "opts=\$RUBY_CONFIGURE_OPTS"
+    [[ -d "\$TMPDIR" ]] && echo "tmpdir-exists=yes"
+  } >"$TEST_DIR/install.log"
+  exit "\${FAKE_INSTALL_RC:-0}"
+  ;;
+global) [[ -n "\${2:-}" ]] && exit 0 ;;
+esac
+exit 0
+EOF
+chmod +x "$RBENV/bin/rbenv"
+
+run() {
+  : >"$TEST_DIR/install.log"
+  (RUBY_VERSION=3.4.7 RUBY_INSTALL_RAILS=false setup_ruby >/dev/null 2>&1)
+}
+logged() { sed -n "s/^$1=//p" "$TEST_DIR/install.log"; }
+
+run
+check "setup succeeds" 0 "$?"
+check "the build tree is on disk, not in tmpfs /tmp" "/var/tmp" "$(dirname "$(logged tmpdir)")"
+check "the build directory exists during the build" "yes" "$(logged tmpdir-exists)"
+check "the build directory is removed afterwards" "no" "$([[ -e "$(logged tmpdir)" ]] && echo yes || echo no)"
+check "docs are not built" "--disable-install-doc " "$(logged opts)"
+
+RUBY_CONFIGURE_OPTS="--with-jemalloc" run
+check "a caller's configure options are kept" "--disable-install-doc --with-jemalloc" "$(logged opts)"
+
+FAKE_INSTALL_RC=1 run
+check "a failed build fails the setup" 150 "$?"
+check "a failed build keeps its log for inspection" "yes" "$([[ -d "$(logged tmpdir)" ]] && echo yes || echo no)"
+
+echo "${passed} passed, ${failed} failed"
+((failed == 0))