support linux_arm as well, e.g. native Docker on Apple Silicon;

--- a/src/Pure/Tools/build_docker.scala	Mon Nov 01 22:33:28 2021 +0100
+++ b/src/Pure/Tools/build_docker.scala	Mon Nov 01 22:37:22 2021 +0100
@@ -12,7 +12,7 @@
   private val default_base = "ubuntu"
   private lazy val default_logic = Isabelle_System.getenv("ISABELLE_LOGIC")
 
-  private val Isabelle_Name = """^.*?(Isabelle[^/\\:]+)_linux\.tar\.gz$""".r
+  private val Isabelle_Name = """^.*?(Isabelle[^/\\:]+)_linux(?:_arm)?\.tar\.gz$""".r
 
   val packages: List[String] =
     List("curl", "less", "libfontconfig1", "libgomp1", "pwgen", "unzip")