Description of problem:
When opening the "Containers" tab in Cockpit's dashboard, logged-in as a member of the 'docker' group.

Version-Release number of selected component:
cockpit-bridge-0.38-1.fc22

Additional info:
reporter:       libreport-2.3.0
backtrace_rating: 4
cmdline:        /usr/bin/cockpit-bridge
crash_function: relay_length
executable:     /usr/bin/cockpit-bridge
kernel:         3.19.0-0.rc7.git3.1.fc22.x86_64
runlevel:       N 5
type:           CCpp
uid:            1000

Truncated backtrace:
Thread no. 1 (9 frames)
 #4 relay_length at src/bridge/cockpithttpstream.c:457
 #5 on_pipe_read at src/bridge/cockpithttpstream.c:533
 #6 ffi_call_unix64 at ../src/x86/unix64.S:76
 #7 ffi_call at ../src/x86/ffi64.c:525
 #8 g_cclosure_marshal_generic_va at gclosure.c:1541
 #9 _g_closure_invoke_va at gclosure.c:831
 #12 dispatch_input at src/common/cockpitpipe.c:269
 #16 g_main_context_iteration at gmain.c:3869
 #17 run_bridge at src/bridge/bridge.c:420