NoCredentialProviders error when running "docker compose up" with AWS ECS integration

I'm getting the following error when I try to run docker compose up to deploy my infrastructure to AWS using Docker's ECS integration. Note that I'm running this on Pop!_OS 21.10, which is based on Ubuntu.

NoCredentialProviders: no valid providers in chain. Deprecated. For verbose messaging see aws.Config.CredentialsChainVerboseErrors

Things I've tried, based on an exhaustive search of SO and other sites:

• Verified the proper format of my ~/.aws/config and ~/.aws/credentials files are formatted correctly, are in the proper place, and have the correct permissions
• Verified that the aws cli works fine
• Verify that AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY, AWS_REGION are all set correctly
• Tried copying the config and credentials to /root/.aws
• Tried setting AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY, AWS_REGION in the root user's environment
• Created /etc/systemd/system/docker.service.d/aws-credentials.conf and populated it with:

[Service]
Environment="AWS_ACCESS_KEY_ID=********************"
Environment="AWS_SECRET_ACCESS_KEY=****************************************"

• Ran docker -l debug compose up (Only extra information it provides is DEBUG deploying on AWS with region="us-east-1"

I'm running out of options. If anyone has any other ideas to try, I'd love to hear it. Thanks!

Update: I've also now tried the following, with no luck:

• Tried setting Environment="AWS_SHARED_CREDENTIALS_FILE=/home/kespan/.aws/credentials
• Tried setting Environment="AWS_SHARED_CREDENTIALS_FILE=/home/kespan/.aws/credentials in /etc/systemd/system/docker.service.d/override.conf
• After remembering my IAM account has MFA enabled, generated a token and added Environment="AWS_SESSION_TOKEN=..." to override.conf

Also to note - each time after I've added/modified files under /etc/systemd/system/docker.service.d/ I've run:

sudo systemctl daemon-reload
sudo systemctl restart docker

Edit:

Here's one of the Dockerfiles (both the scraper and scheduler use an identical Dockerfile):

FROM denoland/deno:alpine
WORKDIR /app
USER deno
COPY deps.ts .
RUN deno cache --unstable --no-check deps.ts
COPY . .
RUN deno cache --unstable --no-check mod.ts
RUN mkdir -p /var/tmp/log
CMD ["run", "--unstable", "--allow-all", "--no-check", "mod.ts"]

Here's my docker-compose (some bits redacted):

version: '3'
services:
  grafana:
    container_name: grafana
    image: grafana/grafana
    ports:
      - "3000:3000"
    volumes:
      - grafana:/var/lib/grafana
    deploy:
      replicas: 1
  scheduler:
    image: scheduler
    x-aws-pull-credentials: "arn..."
    container_name: scheduler
    environment:
      DB_CONNECTION_STRING: "postgres://..."
      SQS_URL: "..."
      SQS_REGION: "us-east-1"
      AWS_ACCESS_KEY_ID: ${AWS_ACCESS_KEY_ID}
      AWS_SECRET_ACCESS_KEY: ${AWS_SECRET_ACCESS_KEY}
    deploy:
      replicas: 1
  scraper:
    image: scraper
    x-aws-pull-credentials: "arn..."
    container_name: scraper
    environment:
      DB_CONNECTION_STRING: "postgres://..."
      SQS_URL: "..."
      SQS_REGION: "us-east-1"
      AWS_ACCESS_KEY_ID: ${AWS_ACCESS_KEY_ID}
      AWS_SECRET_ACCESS_KEY: ${AWS_SECRET_ACCESS_KEY}
    deploy:
      replicas: 1
volumes:
  grafana: