Specifying the Program that Docker Should Launch at Start Time
Sometimes if you are working with a container that gets stuck in a boot loop you need to force it to start a specific application for debugging purposes. We had this problem using the Dropbox container that was build by janeczku.
It works great most of the time, but during one of the Dropbox updates, the container would not start so it would start and stop rapidly. This is a persistent container that comes up each time the server boots so we need to start it into something like `sleep 30m` so that we can run `docker -it exec dropbox /bin/bash` and inspect to see what the problem is.
Modifying the Configuration
In /var/lib/dropbox/<hash>/config.v2.json you will see an XML file similar to the one below, but notably, it has not been pretty printed as we have done for you. Somewhere in the list you will find the “Entrypoint” setting. Our system came with this set as “Entrypoint” : [ “/root/run” ], but /root/run is the script that is exiting and causing the container to boot loop. For our example, we have moved the “Entrypoint” line in our config at the top so you can see it, though yours can be anywhere in the file. Note that the array is the command to run followed by its list of arguments and we set it to /bin/sleep 30m as you can see below so that we can log into the container and get a bash prompt.
Note that the Docker service needs to be stopped before modifying the config file. In our case there was only one container so stopping the service was not an issue; if you have multiple containers then you may need to find a way to modify the config so that Docker will accept it without a restart—and if you do find out how to do that please post it in the comments below.
I hope this works for you, we searched all over the place and had trouble finding these configuration details.
{
"Entrypoint" : [
"/bin/sleep",
"30m"
],
"NetworkSettings" : {
"SecondaryIPAddresses" : null,
"SandboxID" : "cdb37ab22aecc2d6bbde325c515c034f2aa3a2677f39df75c77712ebe2e9c545",
"SecondaryIPv6Addresses" : null,
"SandboxKey" : "/var/run/docker/netns/cdb37ab22aec",
"LinkLocalIPv6Address" : "",
"Bridge" : "",
"HasSwarmEndpoint" : false,
"Service" : null,
"Ports" : null,
"Networks" : {
"bridge" : {
"IPAMOperational" : false,
"EndpointID" : "",
"GlobalIPv6PrefixLen" : 0,
"IPPrefixLen" : 0,
"IPAMConfig" : null,
"IPAddress" : "",
"IPv6Gateway" : "",
"Aliases" : null,
"Links" : null,
"NetworkID" : "237c33c976ab7863c1a869b2f19492ecdd4d820763b7033cc50397147b26d324",
"MacAddress" : "",
"GlobalIPv6Address" : "",
"Gateway" : ""
}
},
"HairpinMode" : false,
"LinkLocalIPv6PrefixLen" : 0,
"IsAnonymousEndpoint" : false
},
"SeccompProfile" : "",
"RestartCount" : 0,
"HostsPath" : "/var/lib/docker/containers/18ea58191394f07d288383cc66c5ea58d99c0dac8c9c5007158b9a2378d6b66e/hosts",
"ExposedPorts" : {
"17500/tcp" : {}
},
"HostnamePath" : "/var/lib/docker/containers/18ea58191394f07d288383cc66c5ea58d99c0dac8c9c5007158b9a2378d6b66e/hostname",
"MountLabel" : "system_u:object_r:svirt_sandbox_file_t:s0:c57,c401",
"HasBeenStartedBefore" : true,
"Labels" : {},
"NoNewPrivileges" : false,
"OpenStdin" : false,
"Hostname" : "18ea58191394",
"Volumes" : {
"/dbox/Dropbox" : {},
"/dbox/.dropbox" : {}
},
"MountPoints" : {
"/dbox/Dropbox" : {
"Spec" : {
"Target" : "/dbox/Dropbox/",
"Type" : "bind",
"Source" : "/data/Dropbox (GPI)/"
},
"RW" : true,
"Destination" : "/dbox/Dropbox",
"Type" : "bind",
"Propagation" : "rprivate",
"Source" : "/data/Dropbox (GPI)",
"Driver" : "",
"Name" : ""
},
"/dbox/.dropbox" : {
"Spec" : {
"Target" : "/dbox/.dropbox",
"Type" : "volume"
},
"Type" : "volume",
"RW" : true,
"Driver" : "local",
"Destination" : "/dbox/.dropbox",
"Source" : "",
"Name" : "eb96b3bab31838496f2ca3ce0b6db476aaba9c16d9b6bc0f7da3d05f1f964120"
}
},
"Env" : [
"DBOX_UID=501",
"DBOX_GID=1011",
"PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
"DEBIAN_FRONTEND=noninteractive"
],
"StdinOnce" : false,
"ArgsEscaped" : true,
"ResolvConfPath" : "/var/lib/docker/containers/18ea58191394f07d288383cc66c5ea58d99c0dac8c9c5007158b9a2378d6b66e/resolv.conf",
"HasBeenManuallyStopped" : false,
"Driver" : "overlay2",
"AttachStdout" : false,
"User" : "root",
"ProcessLabel" : "system_u:system_r:svirt_lxc_net_t:s0:c57,c401",
"Cmd" : null,
"AttachStdin" : false,
"AttachStderr" : false,
"SecretReferences" : null,
"AppArmorProfile" : "",
"ShmPath" : "/var/lib/docker/containers/18ea58191394f07d288383cc66c5ea58d99c0dac8c9c5007158b9a2378d6b66e/shm",
"Image" : "sha256:a8964074d4f6eac2dfdbf03200c4c73d571b1ea7ad8fcb8d99b918642de2f8d2",
"Tty" : false,
"LogPath" : "",
"Domainname" : "",
"WorkingDir" : "/dbox/Dropbox",
"OnBuild" : null,
"Name" : "/dropbox"
}
-Eric