docs: add tarfile support

Signed-off-by: AtomicFS <vojtech.vesely@9elements.com>

Author: AtomicFS

cmd/firmware-action/recipes/config.go

@@ -45,6 +45,7 @@ type CommonOpts struct {
 	// This has an influence on the IASL, GCC and host GCC version that is used to build
 	//   the target. You must match the source level and sdk_version.
 	// Can also be a absolute or relative path to Dockerfile to build the image on the fly.
+	// Can also be a path to tarfile.
 	// NOTE: Updating the sdk_version might result in different binaries using the
 	//   same source code.
 	// ANCHOR: CommonOptsSdkURLExamples
@@ -57,6 +58,7 @@ type CommonOpts struct {
 	//   file://my-image/Dockerfile
 	//   file:///home/user/my-image/Dockerfile
 	//   file:///home/user/my-image/
+	//   file:///home/user/ubuntu-latest.tar
 	// ANCHOR_END: CommonOptsSdkURLExamples
 	// NOTE:
 	//   'file://' path cannot contain '..'

docs/src/firmware-action/offline_usage.md

@@ -1,5 +1,10 @@
 # Offline usage
 
+
+## Occasionally offline machines
+
+This approach is useful for example when people travel. They run `firmware-action` beforehand to pull the necessary containers and then use this method to work offline.
+
 `firmware-action` under the hood uses [dagger](https://docs.dagger.io/) / docker. As such, the configuration contains entry `sdk_url` which specifies the docker image / container to use.
 
 ```admonish example
@@ -31,3 +36,16 @@ It will also be displayed every time `firmware-action` is executed as `INFO` mes
 ```
 
 Simply copy-paste the digest (or image reference) into your configuration file and `firmware-action` will not connect to the internet to fetch a container if one matching is already present.
+
+
+## Always offline machines
+
+Besides running a offline docker registry, there is also a option to use tarballs.
+
+`firmware-action` can import a tarfile and use it.
+
+```admonish example
+~~~json
+"sdk_url": "file:///home/user/my-image/ubuntu-latest.tar"
+~~~
+```