The only “Arcane Sigil Docent” I can find is from Dungeons & Dragons Online, and is not available in the tabletop game (that I can find, but I have read most of the Eberron books).
Usually a Warforged arcanist would take Unarmored Body, rather than Mithril Body, so as to avoid any Arcane Spell Failure at all. A 5% chance of even casting the spell correctly, on top of whatever chance you have of missing or the enemy resisting, is going to hurt a lot.
Note that if you want to be a bit cheesy (in some groups this would be fine, in others it’d be broken; depends on your group), Dragonborn from Races of the Dragon is an amazing template with Warforged, and would be perfect for a Sorcerer (who usually have some relation to dragons to begin with. Dragonborn replaces your racial features, but you retain any Subtypes you have. Since most of the best features of Warforged come from the Living Construct subtype, they keep all of those, losing primarily the composite plating and Slam attack. Since you want to get rid of (parts of) the composite plating anyway, this is a win-win.
The immediate answer to Arcane Spell Failure is usually the twilight armor enhancement (+1 equivalent), found in Player’s Handbook II and the Magic Item Compendium. It can be applied to Warforged composite armor, and reduces Arcane Spell Failure by 10%.
Other good answers, things like Thistledown (Races of the Wild), Feycraft (Dungeon Master’s Guide II), and Githcraft (Dungeon Master’s Guide II), all of which reduce Arcane Spell Failure by 5%, probably cannot reasonably be applied to Warforged composite armor. Even if they can, they’d almost definitely have to be part of the Warforged’s original construction (though a Githcraft Warforged would be awesome).
Again, note that with Unarmored Body or Dragonborn, you could wear a Feycraft-or-Githcraft Mithral Chain Shirt with Thistledown Padding, which has 0% Arcane Spell Failure before putting any magic on it. This is a great armor.
The Spellsword prestige class from Complete Warrior can allow you to ignore 10% of Arcane Spell Failure; combined with twilight, this reduces your Arcane Spell Failure to 0%. I strongly recommend against taking more than one level of Spellsword, however, since it loses a ton of spellcasting.