Changes:
- Adds `ci/Dockerfile` for creating `statusteam/nim-status-client-build:latest` used in builds
- Adds `ci/Jenkinsfile.linux` and `ci/Jenkinsfile.macos` for respective platforms
- Simplifies MacOS signing by adding `scripts/sign-macos-pkg.sh` script
- Makes `Makefile` use `scripts/sign-macos-pkg.sh` to make the DMG
- Makes `APPIMAGE` and `DMG` in `Makefile` modifiable by environment
- Adds `--passL:"-headerpad_max_install_names"` to `NIM_PARAMS` to fix MacOS signing issues
Signed-off-by: Jakub Sokołowski <jakub@status.im>
