.gitignore: ignore *.cer, *.crt, *.key, *.pem and *.img

* .cer, .crt, .key and .pem are typical file name extensions for TLS certificates and keys. They are used when codesigning with openssl and should never be commited in the repo.
* .img is a generic image file. Such files could be used when repacking an ISO to attach cloud-init CIDATA.

.gitignore

@@ -3,5 +3,10 @@ archiso-*.tar.gz*
 work/
 out/
 *.iso
+*.img
+*.cer
+*.crt
+*.key
+*.pem
 user-data
 meta-data