Expanding containers documentation (#1780)

* Expanding containers documentation

* improvements as discussed

---------

Co-authored-by: Pierre Wielders <pierre@wielders.net>

docs/plugin/execution-modes/containers/container.md

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+# Container
+
+!!! note
+	Container support is not enabled by default.
+
+	To enable it, build Thunder with the `-DPROCESSCONTAINERS=ON` CMake option and specify the desired backend using the `-DPROCESSCONTAINERS_XXX=ON` where `XXX` corresponds to the backend you wish to use.
+
+In container mode which is an extension to the out-of-process mode, ThunderPlugin host will run within a containerised environment.  
+
+Thunder supports multiple container integrations through the `ProcessContainer` abstraction mechanism in Thunder core.
+
+If you have only one container integration, that will be the default one. However, if you have enabled multiple container integrations, you must specify the one you want to use.
+
+There are two ways to do this:
+
+- Specify it in the plugin configuration:
+
+	This method allows you to specify which integration to use for the plugin.
+```
+"configuration":{
+   "root":{
+      "mode":"Container",
+      "configuration" : {
+         "containertype":"lxc"
+      }
+   }
+}
+```
+
+- Specify the default integration in the Thunder configuration.
+
+	This method allows you to set a default integration for plugins that do not have a specified container type in their configuration; they will use this default integration.
+```
+"processcontainers" : {
+    "default" : "runc"
+}
+```
+
+## Supported Containers
+
+- [LXC ](https://linuxcontainers.org/)
+- [runc](https://github.com/opencontainers/runc)
+- [crun](https://github.com/containers/crun)
+- [Dobby](https://github.com/rdkcentral/Dobby) (Maintained by RDK)
+- AWC (Maintained Externally)
+
+To run a plugin in a container, a suitable container configuration must already exist. Thunder does not create container configurations dynamically. Thunder will search for existing container configurations in the following locations, in order of priority:
+
+1. `<volatile path>/<callsign>/Container`
+1. `<persistent path>/<callsign>/Container`
+1. `<data path>/<classname>/Container`
+
+For instance, the configuratoin for a plugin with the callsign SamplePlugin might be stored in `/opt/thunder/SamplePlugin/Container` if the persistent path is set to `/opt/thunder` in Thunder configuration.
+
+**Advantages**
+
+* Retains all the benefits of OOP plugins.
+* Enhanced security - Containers have restricted access to the host system including filesystem and device access.
+* Improved resource management - Containers can use cgroups to tightly control and monitor resource usage.
+
+**Disadvantages**
+
+* Maintenance - Requires managing container configurations and appropriately configuring access to resources. (e.g. device nodes)
+* Startup time - Plugin activation may take longer due to the additional overhead of setting up the container environment.

docs/plugin/execution-modes/containers/lxc.md

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+# What is LXC ?
+LXC (Linux Containers) is a lightweight virtualization method that provides an environment similar to a complete Linux system but without the overhead of a full virtual machine. It uses a combination of Linux kernel features like namespaces and cgroups to isolate processes and manage resource allocation..
+
+# Setup in Buildroot/Lithosphere
+
+1. Make sure kernel has all the futures needed for containerization. The easiest way to enable them is to use raspberrypi3_wpe_ml_container_defconfig.
+
+2. Enable containers support in Thunder.
+```
+Thunder -> Extensions -> Process Containers
+```
+
+3. Select RunC as the backend engine for containers. 
+```
+Thunder -> Extensions -> Process Containers -> Containers Backend -> LXC
+```
+
+4. (optional) Enable containers plugin for info about running containers
+```
+Thunder -> Plugins -> ProcessContainers
+```
+
+
+# Setting Up LXC Container
+For demo purposes, we will use the OCDM plugin. To run a containerized ThunderNanoService you will need to create rootfs with all of the dependencies of the nanoservice and ThunderPlugin as executable. For this tutorial rootfs.tar generated by buildroot/lithosphere/yocto will be used as reference roots.
+
+1) Before running the system, create a folder named 'Container' under /rootfs/usr/share/Thunder/OCDM/
+
+2) Create another folder named 'rootfs' under /rootfs/usr/share/Thunder/OCDM/Container
+
+3) Copy the whole rootfs of the sdcard into /rootfs/usr/share/Thunder/OCDM/Container/rootfs
+
+
+# Adjusting Configuration for Thunder
+
+1. Create a file named 'config' under /rootfs/usr/share/Thunder/OCDM/Container and place the below content in it.
+```
+# Template used to create this container: /usr/share/lxc/templates/lxc-download
+# Parameters passed to the template: --no-validate
+# Template script checksum (SHA-1): 273c51343604eb85f7e294c8da0a5eb769d648f3
+# For additional config options, please look at lxc.container.conf(5)
+
+# Uncomment the following line to support nesting containers:
+#lxc.include = /usr/share/lxc/config/nesting.conf
+# (Be aware this has security implications)
+
+lxc.mount.entry = /tmp tmp none bind,optional 0 0
+
+# Distribution configuration
+lxc.include = /usr/share/lxc/config/common.conf
+lxc.include = /usr/share/lxc/config/userns.conf
+lxc.arch = linux32
+
+# Container specific configuration
+# if you see access related issue, disable user/group id settings below
+lxc.idmap = u 0 100000 65536
+lxc.idmap = g 0 100000 65536
+lxc.rootfs.path = dir:/usr/share/Thunder/OCDM/Container/rootfs
+lxc.uts.name = OCDM
+
+# Inherit envionment needed by Thunder nanoservices
+lxc.environment = PATH
+lxc.environment = MESSAGE_DISPATCHER_CONFIG
+
+# Network configuration
+lxc.net.0.type = empty
+#lxc.net.0.type = veth
+#lxc.net.0.link = lxcbr0
+#lxc.net.0.flags = up
+#lxc.net.0.hwaddr = 00:16:XX:XX:XX:XX
+```
+
+2. Comment out below line in /usr/share/lxc/config/common.conf:
+```
+# lxc.seccomp.profile = /usr/share/lxc/config/common.seccomp
+```
+
+3. In plugin configuration (eg. `/etc/Thunder/plugin/OCDM.json` for OCDM) change 
+```"mode": Local``` 
+to 
+```"mode": "Container"```
+
+4. If everything works fine, you should see OCDM working just lie an ordinary OOP plugin.
+
+# Mounting a shared directory between host and LXC container
+
+Most often, we may need to securely share files between the host machine and a container.
+
+Example : File logging from a container to a logging system folder located at the host machine.
+
+1. Create a directory in host.
+```
+mkdir /testshare && chmod 7770 /testshare
+```
+2. Create a directory in lxc container (eg: OCDM container)
+```
+mkdir /usr/share/Thunder/OCDM/Container/rootfs/TestLogging && chmod 7770 /usr/share/Thunder/OCDM/Container/rootfs/TestLogging
+```
+3. Edit container config file (eg: /usr/share/Thunder/OCDM/Container/config)
+```
+lxc.mount.entry = /testshare TestLogging none bind,rw 0 0
+```
+4. UID/GID mapping
+```
+# Container specific configuration
+#lxc.idmap = u 0 100000 65536
+#lxc.idmap = g 0 100000 65536
+
+# Container's UID/GID 0-65535 are mapped to host's 100000-165535,
+# but UID/GID 1000 on the container is mapped to host's UID/GID 1000.
+lxc.idmap = u 0 100000 1000
+lxc.idmap = g 0 100000 1000
+lxc.idmap = u 1000 1000 1
+lxc.idmap = g 1000 1000 1
+lxc.idmap = u 1001 101001 64535
+lxc.idmap = g 1001 101001 64535
+```
+5. Accessing contents from a shared folder and permissions .
+
+    Use case 1 : 
+        Host has created a domain socket within the shared folder.
+        Container running in non-privileged mode (not root mode), trying to write to this domain socket.  
+        In this use case, the write operation will fail, if there is NO write permission for "other" user  (eg: 775 instead of 777).
+        So make sure that the domain socket has been created with the required access right.
+
+## Good To Know
+
+- lxc API to kill a container is blocking, not guarenteed to work and therefore not used
+- If ifconfig is not available in the container use the ip command (e.g. "ip address" to get IP address info for the device)