Why was the Juggernaut, one of the strongest beings on the planet, not able to escape a simple prison? He should have been in the Raft. He didn’t appear to have a collar, not that would have affected him, as he isn’t a mutant. He wasn’t kept immobile, as he was able to grab the lunch tray.
It took some time but I guess the answer as to why the Juggernaut didn't escape (or even try to) is shown in the Super-Duper cut of the movie (And yes, that's what it's named). Here's why:
Juggernaut was put in a highly magnetic cage
In addition to what TheLethalCarrot says in his answer, it was also revealed that The unstoppable Juggernaut was locked in a heavy magnetic field prison. Below is a screenshot from the movie.
When the prisoners are being transported to the Raft, Cable attacks and tries to kill young Firefist. During the commotion, Firefist releases Juggernaut by turning off the magnetic field (as in the picture above), leading to the conclusion that this was what held him at bay.
As the movie is unclear if Juggernaut is actually a mutant, I assume that the mutant collar wasn't put on him (contrary to what I thought before). Juggernaut in the comics is powered by the Crimson gem of Cyttorak which makes his abilities by mystical means rather than by genetic mutation. As this is neither mentioned in the movie, it is inconclusive as to how they came up with the idea to imprison the unstoppable Juggernaut in the first place.
PS: Just a speculation but, this idea could well be given by Professor Xavier, Juggernaut's brother. (Juggernaut mentions Professor X in the movie once leading to the assumption that he is in the background somewhere).
There are several assumptions in your question that may or may not be correct.
a simple prison
Is the Ice Box a simple prison? I don't think so. To start off with it is built underground in a remote snow covered area.
Prisoners are then made to wear collars that remove their mutant powers, wear yellow jumpsuits to stand out and given minimal possessions, looks to be just a a blanket.
As you can see above the guards are also heavily armoured and well equipped to deal with any problems should they occur. The prisoners also live in transparent cells so that the gaurds can see them all the time... this is not a simple prison.
On top of all this Juggernaut is kept in a maximum security cell away from the main prisoners behind a big blast door and down a corridor. Whilst we don't get to see much more than this we can assume this cell has more restrictions to it than what the regular prisoners get.
He didn’t appear to have a collar, not that would have affected him, as he isn’t a mutant.
Whilst I can agree that he doesn't appear to have a collar on which we can tell by comparing the images of him escaping from the convoy truck... and tearing Deadpool in half to when he later fights Colossus.
Of course he could have broken the collar off before breaking out of the truck but we have no way of knowing that given the films events. As for whether he is a mutant or not, I'd imagine that the films events lead us to believe that he is one given it's implied all the prisoners in the Ice Box were mutants but we simply don't know for sure if he is or not.
He wasn’t kept immobile, as he was able to grab the lunch tray.
It's been a while since I've seen the film but there's nothing to tell us he wasn't kept somewhat immobile. Sure he could move around his cell but he may have also been chained up but with long enough chains to move around still.
TLDR: The answer is really they wanted the Juggernaut in the movie, logic be damned.
I ended up rewatching the Super Duper release to get an answer. The Super Duper release has some footage that was cut from the regular release, so there is more information there.
First off the collar. Around 42:20m in the movie, the policeman says "It's a power dampening collar. It shuts down all mutant abilities." This implies it only works on mutants.
We never see the Juggernaut wearing the collar. However, he is always seen wearing his helmet. He is wearing it in the Ice Box in his cell, as well as in the prison transport vehicle, when Fire Fist opens his cage at 1:21:09, his helmet is on and there is no sign of the collar. A metal armband can be seen briefly as he exits the cage.
As Shreedhar pointed out in his answer, the Juggernaut was kept in a magnetic cage on the transport. The Juggernaut is wearing metallic armbands and legbands which he removes from himself after stepping out of the wrecked transport. I suppose we are to assume these four tiny pieces of metal (and possibly his helmet) were strong enough to restrain him in a magnetic field. I guess we just chalk that up to movie logic.
I think its safe to say the Juggernaut is not considered a mutant in this movie, as he is never shown wearing a power damning collar when restrained. At 1:36:12 he also alludes to Xavier being his half-brother, which is more in line with his comic book origin, which is magical.
In the Ice Box, we see Fire Fist give the Juggernaut the tray, who then pulls it in at around 1:04:00. However, we later see it appears he appears to be against the back wall, with bars with red lights in front of the door - presumably to keep him from breaking out. Strangely, the bars are not present on the side wall. There is also a window on the wall with no bars. The movie shows the Juggernaut as invulnerable. So assuming he had the ability he could just bust through that wall with the window. Since he survived the fall from the highway, he could survive a fall from the prison. Also, no collar is visible on him. This shows he has the ability to move forward and back in his cell. Unfortunately, there is nothing given in the movie that explains how he is kept in his cell.
The Ice Box really is just a simple prison. It is made of steel, concrete, and plastic. It gets easily damaged and destroyed by Cable's guns and explosions. The only protection it has is that the mutants inside are wearing power dampening collars.
We are not given any parameters for Juggernaut's strength in this movie, other than what is shown. Obviously, the comic book "once in motion, nothing can stop him" doesnt apply to this movie, as he could have walked out of the prison at any time he chose with the strength he displayed.
In the third X-Men movie, Juggernaut is unstoppable, meaning he can charge straight through walls etc. Later in the movie, there is a mutant who can negate other mutants powers and when he negates Juggernauts powers, Juggernaut runs straight into a wall and knocks himself out because hes just a large, muscular man, rather than an unstoppable mutant. Presumably, Juggernaut's powers work in the same way in Deadpool, which would mean they could have used the same power suppression they use on the rest of the prisoners. As far as the collar is concerned, it could be they built it into the cage, which would explain how he had his powers after he got out of his cage on the transport.