Fix Docker build: clone main branch from PyGEM-Community/PyGEM

[Copilot]

The Docker build job was failing because PyGEM-Community/PyGEM uses main as its default branch, but the Dockerfile and workflow were hardcoded to clone master, which doesn't exist in that repo.

Changes

• docker/Dockerfile: Change ARG PYGEM_BRANCH=master → main
• .github/workflows/docker_pygem.yml: Map this repo's master branch to main when passing PYGEM_BRANCH build arg; add comment explaining the intentional name mismatch

# Before
ARG PYGEM_BRANCH=master

# After
ARG PYGEM_BRANCH=main

# Before
PYGEM_BRANCH=${{ github.ref == 'refs/heads/master' && 'master' || 'dev' }}

# After
# Map this repo's 'master' branch to 'main' in PyGEM-Community/PyGEM (upstream uses 'main')
PYGEM_BRANCH=${{ github.ref == 'refs/heads/master' && 'main' || 'dev' }}