ContainerRegistry: Don't escape slashes in MIME types

[euanh]

Motivation

JSONEncoder escapes slashes in strings, so slashes in MIME types written into JSON metadata objects will be escaped. Runtimes seem to accept this, but do not do it when producing images themselves.

Modifications

Add .withoutEscapingSlashes to the list of JSONEncoder options.

Result

MIME types will not have escaped slashes.

Test Plan

All existing tests continue to pass.