Mount /boot ro by default

See containers/bootc#294
This is particularly motivated by CentOS/centos-bootc-dev#27
because with that suddenly `dnf` will appear to start working
but trying to do anything involving the kernel (i.e. mutating `/boot`)
will end in sadness, and this puts a stop to that.

(This also relates of course to ye olde osbuild#18
 where we want the partitioning setup in the default case
 to come from the container)

Signed-off-by: Colin Walters <walters@verbum.org>

bib/cmd/bootc-image-builder/partition_tables.go

@@ -9,6 +9,11 @@ import (
 const (
  MebiByte = 1024 * 1024 // MiB
  GibiByte = 1024 * 1024 * 1024 // GiB
+ // BootOptions defines the mountpoint optins for /boot
+ // See https://github.com/containers/bootc/pull/341 for the rationale for
+ // using `ro` by default. Briefly it protects against corruption
+ // by non-ostree aware tools.
+ BootOptions = "ro"
 )
 
 var partitionTables = distro.BasePartitionTableMap{
@@ -44,7 +49,7 @@ var partitionTables = distro.BasePartitionTableMap{
  Type: "ext4",
  Mountpoint: "/boot",
  Label: "boot",
- FSTabOptions: "defaults",
+ FSTabOptions: BootOptions,
  FSTabFreq: 1,
  FSTabPassNo: 2,
  },
@@ -89,7 +94,7 @@ var partitionTables = distro.BasePartitionTableMap{
  Type: "ext4",
  Mountpoint: "/boot",
  Label: "boot",
- FSTabOptions: "defaults",
+ FSTabOptions: BootOptions,
  FSTabFreq: 1,
  FSTabPassNo: 2,
  },