The cleric can't just transform their metal cell door to escape
It seems that your concern is that, should a Forge Domain cleric be imprisoned in a jail cell made of metal, they could use Artisan's Blessing to deconstruct the metal bars by transforming them into some innocuous metal object, thus creating a hole in the wall that they can escape through. However, this is not possible. Part of the requirement for using Artisan's Blessing is (emphasis mine):
As part of this ritual, you must lay out metal, which can include coins, with a value equal to the creation.
The requirement to "lay out" the metal implies that the metal to be transformed using this feature cannot be part of a structure or other larger object. Even if the cell door is made of metal, the cleric is not capable of laying out this metal in the manner required by the ritual. Similarly, it wouldn't make sense that the cleric could disable a trebuchet made mostly of wood by transforming the metal hinge into something else. Remember that the feature is called Artisan's Blessing: the intent is to create objects, not destroy things by consuming pieces of them. As long as the jailer doesn't leave random metal objects lying around the cleric's cell, the cleric will be unable to use this feature to create much of anything.
If the cleric could get their cell door off its hinges and lay it down on the floor of their cell, then they could probably transform it, but at that point they have already escaped their cell.
Artisan's Blessing requires more metal than you think
I'll also note that even if the cleric were to acquire some scrap metal large enough to form, say, a miner's pick to help them tunnel through a stone wall, they wouldn't actually be able to create the pick. Why? Because they don't just need enough metal to form the object, they need enough metal to buy it. That is, the metal used in the ritual must have "value equal to the creation". So, if the cleric wants to make a miner's pick out of scrap metal, they will need 2 gp worth of scrap metal in order to do so. How much is the average piece of scrap metal worth? Probably a few copper pieces at most. So it would probably take a large and conspicuous pile of scrap metal to assemble the 2 gp (i.e. 200 cp) worth of value required to form a pick using Artisan's Blessing. 2 gp may be pocket change for the average adventurer, but it's a small fortune for the average prisoner.
If the player of the cleric decides to pursue the "collect random metal objects to make a thing" option, it's going to be your job as DM to decide on the value of the things they collect. As a Forge Domain cleric, it's probably fair to say that the character can accurately appraise the value of a piece of scrap metal, so you can just tell the player something like "you find a rusty metal spoon worth 2 cp" and let them keep a tally of how much value they've accumulated (or let them draw their own conclusions about whether collecting value 2 cp at a time is a viable escape plan).
On the other hand, if the cleric can ask the party rogue to pluck a few gold coins out of a jailer's pocket...