Nuxt.js - Only some process.env values pulling from OS - docker

Question

I'm trying to move some of our environment variables out of the .env file onto the operating system, and it seems to be working with some of the variables but not with others. It's a complicated issue so let me show while telling.
On my local Windows OS I set the variables COLORS_PRIMARY and TEST_LABEL.
I access both in nuxt.config.js and also in a page component.
// ./nuxt.config.js (irrelevant parts removed)
vuetify: {
theme: {
themes: {
light: {
primary: process.env.COLORS_PRIMARY || '#EB4E1A',
}
}
}
},
publicRuntimeConfig: {
TEST_LABEL: process.env.TEST_LABEL,
COLORS_PRIMARY: process.env.COLORS_PRIMARY,
}
// ./pages/index.vue
<template>
<div class="version-label">
label: {{ $config.TEST_LABEL || 'config empty' }} |
color: {{ $config.COLORS_PRIMARY || 'color empty' }}
</div>
</template>
Both of these work fine locally - I can see the their values printed to the page, and in View Page Source, as set in Windows' Environment Variables panel, and Vuetify parses COLORS_PRIMARY correctly, setting a button I have in another component to #FFFF00.
The trouble comes in when I deploy it with Docker to Kubernetes Engine, and then try to pull the values from the Workload's YAML file. At which point only TEST_LABEL pulls through but not COLORS_PRIMARY.
The weird part is that both values are visible in View Page Source and printed to the screen, but the button's background color is #EB4E1A - the fallback value set in nuxt.config.js in the vuetify.theme object.
From all of this I've gathered that for some reason client-side env values work perfectly fine but server-side values (or at least, values accessed in nuxt.config.js) aren't accessible, and I can't figure out why or how to overcome it.
My best guess is that it has something to do with how I dockerize the application, or how I use process.env.whatever in nuxt.config.js, so I rate my configuration probably has to change somehow but I'm not sure what to change it to.
The only resource I could find that almost touches on what I'm on about here is this dev.to article, but he's just talking about OS-based environment variables in general, and I can't figure out how to translate it into something useful to my situation.
Here's my Dockerfile:
FROM node:14.15.4-alpine
# create destination directory
RUN mkdir -p /usr/src/nuxt-app
WORKDIR /usr/src/nuxt-app
# update and install dependency
RUN apk update && apk upgrade
RUN apk add git
# copy the app, note .dockerignore
COPY . /usr/src/nuxt-app/
RUN npm install
RUN npm run build
EXPOSE 8080
ENV NUXT_HOST=0.0.0.0
ENV NUXT_PORT=8080
CMD [ "npm", "start" ]
Basically, what I'm trying to do is replicate how Heroku handles environment variables, in GCP and Kubernetes Engine, so if you don't know how to answer my initial question, I'll equally appreciate any advice on other ways of storing and accessing env vars on GCP+KE.