Pass AWS credentials (IAM role credentials) to code running in docker container

The EC2 metadata service will usually be available from within docker (unless you use a more custom networking setup - see this answer on a similar question).

If your docker network setup prevents it from being accessed, you might use the ENV directive in your Dockerfile or pass them directly during run, but keep in mind that credentials from IAM roles are automatically rotated by AWS.

Amazon does have some mechanisms for allowing containers to access IAM roles via the SDK and either routing/forwarding requests through the ECS agent container or the host. There is way too much to copy and paste, but using --net host is the LEAST recommended option because without additionally filters that allows your container full access to anything it's host has permission to do.

https://docs.aws.amazon.com/AmazonECS/latest/developerguide/task-iam-roles.html

declare -a ENVVARS
declare AWS_ACCESS_KEY_ID AWS_SECRET_ACCESS_KEY AWS_SESSION_TOKEN
get_aws_creds_local () {
   # Use this to get secrets on a non AWS host assuming you've set credentials via some mechanism in the past, and then don't pass in a profile to gitlab-runner because it doesn't see the ~/.aws/credentials file where it would look up profiles
   awsProfile=${AWS_PROFILE:-default}
   AWS_ACCESS_KEY_ID=$(aws --profile $awsProfile configure get aws_access_key_id)
   AWS_SECRET_ACCESS_KEY=$(aws --profile $awsProfile configure get aws_secret_access_key)
   AWS_SESSION_TOKEN=$(aws --profile $awsProfile configure get aws_session_token)
   
}

get_aws_creds_iam () {
  TEMP_ROLE=$(aws sts assume-role --role-arn "arn:aws:iam::123456789012:role/example-role" --role-session-name AWSCLI-Session)
  AWS_ACCESS_KEY_ID=$(echo $TEMP_ROLE | jq -r . Credentials.RoleAccessKeyID)
  AWS_SECRET_ACCESS_KEY=$(echo $TEMP_ROLE | jq -r . Credentials.RoleSecretKey)
  AWS_SESSION_TOKEN=$(echo $TEMP_ROLE | jq -r . Credentials.RoleSessionToken)
}

get_aws_creds_local

get_aws_creds_iam

ENVVARS=("AWS_ACCESS_KEY_ID=$ACCESS_KEY_ID" "AWS_SECRET_ACCESS_KEY=$ACCESS_SECRET_ACCESS_KEY" "AWS_SESSION_TOKEN=$AWS_SESSION_TOKEN")

# passing creds into GitLab runner
 gitlab-runner exec docker stepName $(printf " --env %s" "${ENVVARS[@]}")

# using creds with a docker container
docker run -it --rm $(printf " --env %s" "${ENVVARS[@]}") amazon/aws-cli sts get-caller-identity