feat: Add config option to allow user to select the default image source location

Signed-off-by: Christopher Phillips <christopher.phillips@anchore.com>

README.md

@@ -110,9 +110,7 @@ The above output includes only software that is visible in the container (i.e.,
 syft <image> --scope all-layers
 ```
 
-
-
-## Supported sources
+### Supported sources
 
 Syft can generate a SBOM from a variety of sources:
 
@@ -141,7 +139,13 @@ file:path/to/yourproject/file            read directly from a path on disk (any
 registry:yourrepo/yourimage:tag          pull image directly from a registry (no container runtime required)
 ```
 
-#### Default Cataloger Configuration by scan type
+If an image source is not provided and cannot be detected from the given reference it is assumed the image should be pulled from the Docker daemon.
+If docker is not present, then the Podman daemon is attempted next, followed by reaching out directly to the image registry last.
+
+
+This default behavior can be overridden with the `default-image-pull-source` configuration option (See [Configuration](https://github.com/anchore/syft#configuration) for more details).
+
+### Default Cataloger Configuration by scan type
 
 ##### Image Scanning:
 - alpmdb
@@ -179,7 +183,7 @@ registry:yourrepo/yourimage:tag          pull image directly from a registry (no
 - conan
 - hackage
 
-#### Non Default:
+##### Non Default:
 - cargo-auditable-binary
 
 ### Excluding file paths
@@ -393,7 +397,7 @@ Certificate subject:  test.email@testdomain.com
 Certificate issuer URL:  https://accounts.google.com
 ```
 
-#### Local private key support
+### Local private key support
 
 To generate an SBOM attestation for a container image using a local private key:
 ```
@@ -436,6 +440,10 @@ file: ""
 # same as SYFT_CHECK_FOR_APP_UPDATE env var
 check-for-app-update: true
 
+# allows users to specify which image source should be used to generate the sbom
+# valid values are: registry, docker, podman
+default-image-pull-source: ""
+
 # a list of globs to exclude from scanning. same as --exclude ; for example:
 # exclude:
 #   - "/etc/**"