fix: Podman image inspect digest for local images

When using `docker://` prefix to use a base image from the local docker
daemon, Jib expects the `{{.Id}}` field from the `docker image inspect`
output to contain a valid image digest prefixed with `sha256:`. However,
with Podman this is not the case and such builds fail with an "Invalid
digest" exception, because Podman returns only the 64-char hash value in
the `{{.Id}}` field without the `sha256:` prefix.

`CliDockerClient` already has the functionality to deal with the hashes
without the `sha256:` prefix. A new method, `fromDigestOrgHash` has
beend added to `DescriptorDigest` which first checks for the digest
prefix and then checks for the hash. This allows full backwards
compatibility and accepts the Podman version of the image Ids.

jib-core/src/main/java/com/google/cloud/tools/jib/api/DescriptorDigest.java

@@ -79,6 +79,15 @@ public static DescriptorDigest fromDigest(String digest) throws DigestException
     return new DescriptorDigest(hash);
   }
 
+  public static DescriptorDigest fromDigestOrHash(String digestOrHash) throws DigestException {
+      if (digestOrHash.matches(DIGEST_REGEX)) {
+          return fromDigest(digestOrHash);
+      } else if (digestOrHash.matches(HASH_REGEX)) {
+          return fromHash(digestOrHash);
+      }
+      throw new DigestException("Invalid digest or hash: " + digestOrHash);
+  }
+
   private DescriptorDigest(String hash) {
     this.hash = hash;
   }

jib-core/src/main/java/com/google/cloud/tools/jib/docker/CliDockerClient.java

@@ -81,7 +81,7 @@ public long getSize() {
 
     @Override
     public DescriptorDigest getImageId() throws DigestException {
-      return DescriptorDigest.fromDigest(imageId);
+      return DescriptorDigest.fromDigestOrHash(imageId);
     }
 
     /**