docs: add FAQ entries for Docker (#411)

Add FAQ entries for running PGAdapter as a Docker container.
- Forgetting the `-x` option will cause the connection to be closed
  unexpectedly.
- Slightly older versions of the Docker engine can cause the error
  'There is insufficient memory for the Java Runtime Environment to continue.'

The memory error was reported in #405

docs/faq.md

@@ -50,6 +50,34 @@ java -jar pgadapter.jar -p my-project -i my-instance -d my-database \
      -r="minSessions=200;maxSessions=1600;numChannels=16"
 ```
 
+## Docker
+
+### Why am I getting the error 'server closed the connection unexpectedly'?
+Why am I getting the error `error: connection to server at "localhost" (::1), port 5432 failed: server closed the connection unexpectedly`?
+
+You need to start PGAdapter with the command line argument `-x` to allow connections from other hosts
+than localhost. Example:
+
+```shell
+docker run -p 5432:5432 \
+  gcr.io/cloud-spanner-pg-adapter/pgadapter \
+  -p my-project -i my-instance -x
+```
+
+PGAdapter will by default only allow connections from the same host as where PGAdapter is running.
+This is to prevent accidentally giving access to unauthorized users to a Cloud Spanner database when
+you start PGAdapter on a host without a firewall, or without a firewall rule that blocks incoming
+traffic on port 5432. A connection from the host machine to a Docker container is not seen as a
+connection coming from localhost.
+
+### Why am I getting the error 'There is insufficient memory for the Java Runtime Environment to continue.'?
+This error is caused by a change in the base image that is used for the Docker image for PGAdapter
+that is incompatible with Docker versions prior to version [20.10.10](https://github.com/moby/moby/releases/tag/v20.10.10).
+The added support for the clone3 syscall that is included in version 20.10.10 is required.
+
+Upgrading your Docker engine to at least version 20.10.10 should fix this problem.
+
+See also https://github.com/adoptium/containers/issues/215#issuecomment-1142046045 for more details.
 
 ## DDL
 See [DDL options with PGAdapter](./ddl.md) for background information on the DDL options when starting