Add command for packing the ZODB [6.1.x] (#179)

Fixes #104 

I have locally tested with a plain container, using ZEO and also using
Relstorage with Postgres and it works.

But I would like someone else to test with Relstorage, as I don't have
any experience with it.

news/104.feature

@@ -0,0 +1 @@
+Provide command to pack ZODB. @erral

skeleton/docker-entrypoint.sh

@@ -163,6 +163,14 @@ elif  [[ "$1" == "run" ]]; then
   exec $sudo $VENVBIN/zconsole run etc/${CONF} "${@:2}"
 elif  [[ "$1" == "addzopeuser" ]]; then
   exec $sudo $VENVBIN/addzopeuser -c etc/${CONF} "${@:2}"
+elif  [[ "$1" == "pack" ]]; then
+  if [[ -v ZEO_ADDRESS ]]; then
+    exec $sudo $VENVBIN/zeopack "${@:2}" ${ZEO_ADDRESS}
+  elif [[ -v RELSTORAGE_DSN ]]; then
+    exec $sudo $VENVBIN/zodbpack "${@:2}" etc/relstorage_pack.conf
+  else
+    exec $sudo $VENVBIN/zconsole run etc/${CONF} /app/scripts/pack.py
+  fi
 else
   # Custom
   exec "$@"

skeleton/etc/relstorage_pack.conf

@@ -0,0 +1,6 @@
+<relstorage>
+  pack-gc true
+  <postgresql>
+    dsn $(RELSTORAGE_DSN)
+  </postgresql>
+</relstorage>

skeleton/scripts/pack.py

@@ -0,0 +1,2 @@
+from Zope2 import DB
+DB.pack()