Update linux install instructions

ci-install/linux/README.md

@@ -44,11 +44,13 @@ check_interval = 10
   cache_dir = "/local/cache"
   [runners.docker]
     tls_verify = false
-    image = "python:latest"
+    image = "continuumio/conda-concourse-ci"
     privileged = false
     disable_cache = false
-    volumes = ["/local/cache"]
+    volumes = ["/var/run/docker.sock:/var/run/docker.sock", "/local/cache"]
     extra_hosts = ["www.idiap.ch:172.31.100.235"]
+  [runners.cache]
+     Insecure = false
 
 [[runners]]
   name = "docker-build"