v0.7.2

This is a minor release of the spec which should be fully backwards-compatible but extends functionality in several ways and loosens some restrictions.

Spec changes:

• The requirement for the metadata service has been downgraded to a SHOULD. This requirement necessitated a daemon which seemed burdensome for implementations.
• Added a SHOULD requirement for Linux ACEs to provide a basic /etc/hosts if none already exists in application filesystems
• The requirement for exec fields in the app schema to be non-empty was removed. An ACE is permitted to override/replace this section and so this provides greater flexibility when generating images, particularly when converting from other image types to ACI.
• Added mode, uid, and gid parameters to empty volumes. This allows setting which permissions are applied to empty volumes.
• Changed the definition of the executable path in app.exec to be PATH-dependent. The procedure for an ACE to locate the executable path mimics that of the shell (as described in man 3 exec). This means that executable paths are no longer required to be absolute.

Other changes:

• Improved manifest parsing errors - when provided manifests are invalid JSON, the erroneous line and column number will now be highlighted in the produced error
• The discovery code now includes per-host HTTP headers, allowing authentication during discovery
• Added several new helper functions for initializing memory and CPU isolator types
• Added several helpers to work with LinuxCapabilitiesSet schema types
• Refactored the MakeQueryString helper to centralise the parsing of different comma-separated label/value strings used in several places (for volumes, ports, mountpoints, etc). This also now escapes values when parsing, allowing values with special URL characters like "+" or "&"
• Fixed a nil-pointer dereference in the schema Volume type's String() method