config-linux: Make linux.seccomp.syscalls OPTIONAL

Before this commit, linux.seccomp.sycalls was required, but we didn't
require an entry in the array.  That means '"syscalls": []' would be
technically valid, and I'm pretty sure that's not what we want.

If it makes sense to have a seccomp property that does not need
syscalls entries, then syscalls should be optional (which is what this
commit is doing).

If it does not makes sense to have an empty/unset syscalls then it
should be required and have a minimum length of one.

Before 652323c (improve seccomp format to be more expressive,
2017-01-13, opencontainers#657), syscalls was omitempty (and therefore more
optional-feeling, although there was no real Markdown spec for seccomp
before 3ca5c6c, config-linux.md: fix seccomp, 2017-03-02, opencontainers#706, so
it's hard to know).  This commit has gone with OPTIONAL, because a
seccomp config which only sets defaultAction seems potentially valid.

The SCMP_ACT_KILL example is prompted by:

On Tue, Apr 25, 2017 at 01:32:26PM -0700, David Lyle wrote [1]:
> Technically, OPTIONAL is the right value, but unless you specify the
> default action for seccomp to be SCMP_ACT_ALLOW the result will be
> an error at run time.
>
> I would suggest an additional clarification to this fact in
> config-linux.md would be very helpful if marking syscall as
> OPTIONAL.

I've phrased the example more conservatively, because I'm not sure
that SCMP_ACT_ALLOW is the only possible value to avoid an error.  For
example, perhaps a SCMP_ACT_TRACE default with an empty syscalls array
would not die on the first syscall.  The point of the example is to
remind config authors that without a useful syscalls array, the
default value is very important ;).

Also add the previously-missing 'required' property to the seccomp
JSON Schema entry.

[1]: opencontainers#768 (comment)

Signed-off-by: W. Trevor King <wking@tremily.us>

Author: wking

config-linux.md

@@ -610,7 +610,10 @@ The following parameters can be specified to setup seccomp:
     * `SCMP_ARCH_PARISC`
     * `SCMP_ARCH_PARISC64`
 
-* **`syscalls`** *(array of objects, REQUIRED)* - match a syscall in seccomp.
+* **`syscalls`** *(array of objects, OPTIONAL)* - match a syscall in seccomp.
+
+    While this property is OPTIONAL, some values of `defaultAction` are not useful without `syscalls` entries.
+    For example, if `defaultAction` is `SCMP_ACT_KILL` and `syscalls` is empty or unset, the kernel will kill the container process on its first syscall.
 
     Each entry has the following structure:
 

schema/config-linux.json

@@ -251,7 +251,10 @@
                             "$ref": "defs-linux.json#/definitions/Syscall"
                         }
                     }
-                }
+                },
+                "required": [
+                    "defaultAction"
+                ]
             },
             "sysctl": {
                 "id": "https://opencontainers.org/schema/bundle/linux/sysctl",

specs-go/config.go

@@ -484,7 +484,7 @@ type WindowsNetworkResources struct {
 type LinuxSeccomp struct {
 	DefaultAction LinuxSeccompAction `json:"defaultAction"`
 	Architectures []Arch             `json:"architectures,omitempty"`
-	Syscalls      []LinuxSyscall     `json:"syscalls"`
+	Syscalls      []LinuxSyscall     `json:"syscalls,omitempty"`
 }
 
 // Arch used for additional architectures