Support .devcontainer/Dockerfile for license headers check

[shahmishal]

Run curl -s https://raw.githubusercontent.com/swiftlang/github-workflows/refs/heads/main/.github/workflows/scripts/check-license-header.sh | bash
  curl -s https://raw.githubusercontent.com/swiftlang/github-workflows/refs/heads/main/.github/workflows/scripts/check-license-header.sh | bash
  shell: /usr/bin/bash -e {0}
  env:
    PROJECT_NAME: VS Code Swift
** ERROR: Unsupported file extension Dockerfile for file (exclude or update this script): .devcontainer/Dockerfile
bash: line 74: expected_file_header: unbound variable

[FranzBusch]

We normally expect no license header in Dockerfiles and so far we have ignored those in .licenseignore files. I would personally say we don't need to add support for Dockerfiles here