Footfall Dock Code
The locked container sits in a nook on the Void Dock (Ro-Alpha) map in the Footfall hub. It takes a four-digit code, and the clues come from a "Strange Memo" you loot off a body near the center of the docks. The memo lists four hints, one per digit.
The code is 3 - 5 - 4 - 1.
Enter it and the container opens. If you want to understand where each digit comes from, or you are reading the clues yourself in-game, here is how the memo resolves.
Per-Digit Derivation
Digit 3 points to the Holy Book by the altar in the north-east corner. It is Volume Three, so the first digit is 3.
Digit 5 comes from the customs functionary standing near the red skull flag on the east side. He is a rank-Five functionary, which gives you 5.
Digit 4 is the "guess it yourself" clue, and it works by elimination. Each number button on the lock can be pressed only once, so after you have entered the other three digits, 4 is the only one left.
Digit 1 is tied to the hangar by the window in the north-west corner, where a series of "ones" is marked on the wall. That gives you the final 1.
Read in that order, the four clues spell out 3, then 5, then 4, then 1.
Reward
The container holds several early-chapter armor pieces. They are solid for where you are in the campaign rather than top-tier gear, but worth grabbing while you are passing through Footfall and kitting out the party.
For the rest of the codes and gated encounters across the game, the puzzle and walkthrough hub has the full list.