Switch to Debian:Slim from Alpine for the docker image (#7839)

As mentioned in #7397, switching to a debian base should help with multi-arch work to save time on compiling. This is unashamedly based on #6373, but without the extra functionality. Switch python version back to generic 3.7 to always pull the latest. Essentially, keeping this as small as possible. The image is bigger though unfortunately.

docker/start.py

@@ -120,7 +120,7 @@ def generate_config_from_template(config_dir, config_path, environ, ownership):
 
     if ownership is not None:
         subprocess.check_output(["chown", "-R", ownership, "/data"])
-        args = ["su-exec", ownership] + args
+        args = ["gosu", ownership] + args
 
     subprocess.check_output(args)
 
@@ -172,8 +172,8 @@ def run_generate_config(environ, ownership):
         # make sure that synapse has perms to write to the data dir.
         subprocess.check_output(["chown", ownership, data_dir])
 
-        args = ["su-exec", ownership] + args
-        os.execv("/sbin/su-exec", args)
+        args = ["gosu", ownership] + args
+        os.execv("/usr/sbin/gosu", args)
     else:
         os.execv("/usr/local/bin/python", args)
 
@@ -189,7 +189,7 @@ def main(args, environ):
         ownership = "{}:{}".format(desired_uid, desired_gid)
 
     if ownership is None:
-        log("Will not perform chmod/su-exec as UserID already matches request")
+        log("Will not perform chmod/gosu as UserID already matches request")
 
     # In generate mode, generate a configuration and missing keys, then exit
     if mode == "generate":
@@ -236,8 +236,8 @@ running with 'migrate_config'. See the README for more details.
 
     args = ["python", "-m", synapse_worker, "--config-path", config_path]
     if ownership is not None:
-        args = ["su-exec", ownership] + args
-        os.execv("/sbin/su-exec", args)
+        args = ["gosu", ownership] + args
+        os.execv("/usr/sbin/gosu", args)
     else:
         os.execv("/usr/local/bin/python", args)