First build succeeded.

Fockerfile

@@ -2,5 +2,5 @@ base: freebsd-12.1
 
 steps:
   - run: |
-      pkg install python3 && \
-      pkg install py37-pip
+      pkg -y install -y python3 && \
+      pkg -y install -y py37-pip

image.py

@@ -1,6 +1,8 @@
 from .zfs import *
 import os
 import yaml
+from .steps import create_step
+from .snapshot import new_snapshot
 
 
 def process_step(step, name):
@@ -18,13 +20,37 @@ def process_steps(steps, name):
 
 def build(spec):
     if 'base' not in spec:
-        raise ValueError('Missing base specification')
+        raise ValueError('Missing base in specification')
+
+    if 'steps' not in spec:
+        raise ValueError('Missing steps in specification')
+
     base = spec['base']
-    base = zfs_snapshot_by_tag_or_sha256(base)
+    base, base_sha256 = zfs_snapshot_by_tag_or_sha256(base)
 
     root = '/'.join(base.split('/')[:-1])
     print('base:', base, 'root:', root)
 
+    steps = spec['steps']
+    if not isinstance(steps, list):
+        steps = [ steps ]
+
+    for st in steps:
+        st = create_step(st)
+        st_sha256 = st.hash(base_sha256)
+        for pre in range(7, 64):
+            name = root + '/' + st_sha256[:pre]
+            if not zfs_exists(name):
+                break
+        snap_name = new_snapshot(base, lambda: st.execute(zfs_mountpoint(name)), name)
+        feed = {
+            'focker:sha256': st_sha256
+        }
+        zfs_tag(name, feed)
+        # zfs_tag(snap_name, feed)
+        base = snap_name
+        base_sha256 = st_sha256
+
 
 def command_image_build(args):
     fname = os.path.join(args.focker_dir, 'Fockerfile')

jail.py

@@ -1,4 +1,6 @@
 import subprocess
 
 def jail_run(path, command):
-    subprocess.check_output(['jail', '-c', 'path=' + path, 'command', '/bin/sh', '-c', command)
+    command = ['jail', '-c', 'interface=lo1', 'ip4.addr=127.0.1.0', 'path=' + path, 'command', '/bin/sh', '-c', command]
+    print('Running:', ' '.join(command))
+    subprocess.check_output(command)

snapshot.py

@@ -1,4 +1,4 @@
-from zfs import *
+from .zfs import *
 
 
 def new_snapshot(base, fun, name):
@@ -15,4 +15,6 @@ def new_snapshot(base, fun, name):
         zfs_run(['zfs', 'destroy', name])
         raise
     zfs_run(['zfs', 'set', 'readonly=on', name])
-    zfs_run(['zfs', 'snapshot', name + '@1'])
+    snap_name = name + '@1'
+    zfs_run(['zfs', 'snapshot', snap_name])
+    return snap_name

steps.py

@@ -1,6 +1,6 @@
 import hashlib
 import json
-from jail import jail_run
+from .jail import jail_run
 import shutil
 
 
@@ -31,7 +31,7 @@ class RunStep(object):
 
     def execute(self, path):
         spec = self.spec
-        if isinstance(spec, list)
+        if isinstance(spec, list):
             spec = ' && ' .join(self.spec)
         jail_run(path, spec)
 

zfs.py

@@ -29,7 +29,7 @@ def zfs_snapshot_by_tag_or_sha256(s):
         raise ValueError('Reference not found: ' + s)
     if len(lst) > 1:
         raise ValueError('Ambiguous reference: ' + s)
-    return lst[0][3]
+    return (lst[0][3], lst[0][0])
 
 
 def zfs_clone(name, target_name):
@@ -44,6 +44,16 @@ def zfs_exists(name):
     return True
 
 
+def zfs_tag(name, props):
+    for (k, v) in props.items():
+        zfs_run(['zfs', 'set', k + '=' + v, name])
+
+
+def zfs_mountpoint(name):
+    lst = zfs_parse_output(['zfs', 'list', '-o', 'mountpoint', '-H', name])
+    return lst[0][0]
+
+
 def zfs_init():
     poolname = zfs_parse_output(['zfs', 'list', '-H', '/'])
     if len(poolname) == 0: