That the containers are metal seems obvious
"the gray metallic containers" (page 11, To Your Scattered Bodies Go)
"the mushroom-shaped stones, the straps, the gray metallic containers" (page 12, To Your Scattered Bodies Go)
"rectangular container of gray metal" (page 15, To Your Scattered Bodies Go )
The bodies of the dead are used a source of raw materials
"convert human skin into leather"
"Strips of leather, undoubtedly human skin"
"The dead had to be taken to the rendering factory up in the hills, since their fat was used to make glycerin"
When someone dies their grail stops working and becomes just an object.
"They gave the slave enough to eat because the grail of a dead slave became useless"
But I don't recall any mention of why the grails are not melted down and refined into metal tools and weapons. The search for raw materials is a major plot point of The Fabulous Riverboat (1971).
When resurrected one always gets a new grail
"Unlike the first time he had been resurrected, he was not weak and bewildered... There was nothing at hand except the grail that always appeared with a resurrectee and the pile of towels of various sizes"
A few minor mentions implied that extra towels are available as resources
"the chain of white towels"
But using the metal of the grail to create weapons does not occur.
"Cyrano had take so long to reconcile himself to the fact that he would never hold a metal sword in his hand again. Then he heard about the meteorite and came here"
Is there a canon reason that grails are not exploited as a source of metal? While they might not provide sufficient materials for a river boat, they could be used to make tools and weapons.