[gitignore] Ignore all auto-generated Dockerfiles (#4195)

* [gitignore] Ignore all auto-generated Dockerfiles
- Simplify gitignore to ignore all known directories with generated Dockerfiles
- Rename Barefoot and Cavium saiserver Dockerfiles to follow same convention as other platforms

Signed-off-by: Danny Allen <daall@microsoft.com>

.gitignore

@@ -109,30 +109,9 @@ src/thrift/*
 !src/thrift/Makefile
 
 # Autogenerated Dockerfiles
-sonic-slave/Dockerfile
-sonic-slave-stretch/Dockerfile
-sonic-slave-jessie/Dockerfile
-dockers/docker-base/Dockerfile
-dockers/docker-base-stretch/Dockerfile
-dockers/docker-config-engine/Dockerfile
-dockers/docker-config-engine-stretch/Dockerfile
-dockers/docker-database/Dockerfile
-dockers/docker-dhcp-relay/Dockerfile
-dockers/docker-fpm-frr/Dockerfile
-dockers/docker-fpm-gobgp/Dockerfile
-dockers/docker-fpm-quagga/Dockerfile
-dockers/docker-lldp-sv2/Dockerfile
-dockers/docker-orchagent/Dockerfile
-dockers/docker-platform-monitor/Dockerfile
-dockers/docker-router-advertiser/Dockerfile
-dockers/docker-snmp-sv2/Dockerfile
-dockers/docker-teamd/Dockerfile
-dockers/docker-sonic-mgmt/Dockerfile
-dockers/docker-sonic-telemetry/Dockerfile
-platform/*/docker-saiserver-*/Dockerfile
-platform/*/docker-syncd-*/Dockerfile
-platform/*/docker-syncd-*-rpc/Dockerfile
-platform/vs/docker-sonic-vs/Dockerfile
+sonic-slave*/Dockerfile
+dockers/*/Dockerfile
+platform/*/docker-*/Dockerfile
 
 # Installer-related files and directories
 installer/x86_64/platforms/