overriding environment variable in custom container

[#1]

Hello

When creating a custom engine , environment variable are set when they do not exists but cannot figure out how override an existing one :

Dockerfile :

...

ENV foo=bar <= will be set in engine

ENV   JOB_MAXIMUM_MINUTES=7777 <= will not override the default one 

any help appreciated .

[#2]

Environmental variables are currently set at runtime. You can override the
defaults global in Admin > Engine panel or within a project in Settings >
Environment.

Best,
Tristan