infra/image/shdefaults: Add SYS_PTRACE to CAP_DEFAULTS

Debugging is now enabled by default in the containers that are
generated with container_create. "+SYS_PTRACE" has been added to
CAP_DEFAULTS in shdefaults for this.

infra/image/shdefaults

@@ -5,4 +5,5 @@
 # Use +CAP to add the capability and -CAP to drop the capability.
 CAP_DEFAULTS=(
     "+DAC_READ_SEARCH"  # Required for SSSD
+    "+SYS_PTRACE"       # Required for debugging
 )