Define Bedrock Connect version via BRC_VERSION environment variable.

Currently, the Docker image has the Bedrock Connect version hardcoded into the image. This makes it necessary to always build a new Docker image promptly when a new version of the Bedrock Connect is released.

With the new variable BRC_VERSION the desired version can be defined at the start of the Docker image itself. If the variable is not set, the latest Bedrock Connect version is automatically used when the Docker image is started.