Add initial internals doc

docs/internals.md

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+# Internals
+
+This page documents internal code details and design decisions.
+
+The resulting Docker image contains the following:
+
+* Base linux distribution - this provides standard Linux libraries (such as "glibc") and utilities (such as "ls" and "grep") required by MQ
+* MQ installation (under `/opt/mqm`)
+* Three additional programs, to enable running in a containerized environment:
+   - `runmqserver` - The main process, which creates and runs a queue manager
+   - `chkmqhealthy` - Checks the health of the queue manager.  This can be used by (say) a Kubernetes liveness probe.
+   - `chkmqready` - Checks if the queue manager is ready for work.  This can be used by (say) a Kubernetes readiness probe.
+
+## runmqserver
+The `runmqserver` command has the following responsibilities:
+
+* Checks license acceptance
+* Sets up `/var/mqm`
+    - MQ data directory needs to be set up at container creation time.  This is done using the `crtmqdir` utility, which was introduced in MQ V9.0.3
+    - It assumes that a storage volume for data is mounted under `/mnt/mqm`.  It creates a sub-directory for the MQ data, so `/var/mqm` is a symlink which resolves to `/mnt/mqm/data`.  The reason for this is that it's not always possible to change the ownership of an NFS mount point directly (`/var/mqm` needs to be owned by "mqm"), but you can change the ownership of a sub-directory.
+* Acts like a daemon
+    - Handles UNIX signals, like SIGTERM
+    - Works as PID 1, so is responsible for [reaping zombie processes](https://blog.phusion.nl/2015/01/20/docker-and-the-pid-1-zombie-reaping-problem/)
+* Creating and starting a queue manager
+* Configuring the queue manager, by running any MQSC scripts found under `/etc/mqm`