Pin ruby-2.7.2 image, add ruby-2.7

The ruby-2.7 tags will always contain the latest
patch released for that minor version.

This allows us to ship patches more easily but
retains backwards compatibility with existing
repos asking for 2.7.2.

patches/ruby/2.7.4/debug-segfault.patch

@@ -0,0 +1,25 @@
+diff --git a/class.c b/class.c
+index c866d1d727..37ff3c5ade 100644
+--- a/class.c
++++ b/class.c
+@@ -27,6 +27,7 @@
+ #include "ruby/st.h"
+ #include "constant.h"
+ #include "vm_core.h"
++#include "vm_debug.h"
+ #include "id_table.h"
+ #include <ctype.h>
+ 
+@@ -119,6 +120,12 @@ rb_class_foreach_subclass(VALUE klass, void (*f)(VALUE, VALUE), VALUE arg)
+     while (cur) {
+ 	VALUE curklass = cur->klass;
+ 	cur = cur->next;
++
++	if (curklass == 0) {
++		fprintf(stderr, "=== Detected NULL subclass:\n");
++		dp(curklass);
++	}
++
+ 	f(curklass, arg);
+     }
+ }