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 Environment variables cannot be used to schedule containers to meet the security policy requirements. Environment variables are used to pass configuration data to the containers, not to control where they run1. To schedule containers to run on separate nodes in a Swarm cluster, you need to use node labels and service constraints23. Node labels are key-value pairs that you can assign to nodes to organize them into groups4. Service constraints are expressions that you can use to limit the nodes where a service can run based on the node labels. For example, you can label some nodes as env=dev and others as env=prod, and then use the constraint --constraint node.labels.env==dev or --constraint node.labels.env==prod when creating a service to ensure that it runs only on the nodes with the matching label. References:

• 1: Environment variables in Compose | Docker Docs
• 2: Deploy services to a swarm | Docker Docs
• 3: How to use Docker Swarm labels to deploy containers on specific nodes
• 4: Manage nodes in a swarm | Docker Docs
• [5]: Swarm mode routing mesh | Docker Docs
• [6]: Docker Swarm - How to set environment variables for tasks on various nodes

= The purpose of Docker Content Trust is to sign and verify image tags using digital signatures for data sent to and received from remote Docker registries12. This allows client-side or runtime verification of the integrity and publisher of specific image tags, ensuring the provenance and security of container images34. References:

• 1: Content trust in Docker | Docker Docs
• 2: Docker Content Trust: What It Is and How It Secures Container Images
• 3: Docker Content Trust in Azure Pipelines - Azure Pipelines
• 4: 4.5 Ensure Content trust for Docker is Enabled | Tenable®

While creating a Dockerfile for each environment is a possible solution, it is not the most efficient or scalable way to provision configuration to containers at runtime. Docker provides several mechanisms to inject configuration into containers at runtime, such as environment variables, command line arguments, Docker secrets for sensitive data, or even configuration files mounted as volumes. These methods allow the same Docker image to be used across multiple environments, promoting immutability and consistency across your deployments. Creating a separate Dockerfile for each environment would mean maintaining multiple versions of the Dockerfile, which could lead to inconsistencies and is generally not a recommended practice.

Docker’s bridge network allows containers connected to the same bridge network to communicate, while providing isolation from containers that aren’t connected to that bridge network1. By using the network attach command, you can connect a container to the bridge network, making it reachable from the host’s network12. However, it’s important to note that containers on the default bridge network can only access each other by IP addresses, unless you use the --link option, which is considered legacy2. For better isolation and automatic DNS resolution between containers, it’s recommended to use user-defined bridge networks12.