fix signing key URIs for local files

[iammattcoleman]

Problem description

When using a local file for a repository signing key, a broken URI syntax currently has to be used in order to provide a relative path: valid file:// URIs will always point to an absolute path: Kiwi interprets file://example.key as pointing to the file /example.key.

Invalid syntax can currently be used to provide a relative path. For example, I am currently using file:image-desc/repository.key to use a key file within my image description directory (image-desc).

Expected behaviour

1. Only valid URIs should be usable with <signing key="..."/>: file:relative/path/to/file should not be accepted as valid syntax. According to RFC8089, file: URIs only support absolute paths.

   The following are examples of valid file URIs:

   • file:/path/to/file
   • file:///path/to/file

   Using two only two leading slashes (file://path/to/file) is not valid syntax.

2. Similar to <include>, support for this:// should be added to enable use of key files stored in the image description directory.

Steps to reproduce the behaviour

1. Add <signing key="file://example.key"/> to a repository in an existing description
2. Run a build
3. The build will fail due to not being able to find /example.key.
4. Change the key path to file:nonexistent/example.key in the XML
5. Run a build
6. The build will fail due to not being able to find nonexistent/example.key within the current working directory.

OS and Software information

• KIWI version: 9.24.14
• Operating system: Ubuntu 20.04
• OBS version: n/a