Add enable-admin-container helper to control container with instructions

[tjkirch]

Fixes #402

The hack to show the motd in sh is fun; see the comments in the Dockerfile. I figure it still makes sense to have a bashrc and point to that with ENV, even if we/ssm-agent are not using bash right now.

---

Testing done:

On login, we now see a helpful message:

$ aws ssm start-session --target i-00de67cefaed0840e                                                        [17:31:11]

Starting session with SessionId: tjk-03f919adca34f4e66
Welcome to Thar's control container!

This container gives you access to the Thar API, which in turn lets you inspect
and configure the system.  You'll probably want to use the `apiclient` tool for
that; for example, to inspect the system:

   apiclient -u /settings

You can run `apiclient --help` for usage details, and check the main Thar
documentation for descriptions of all settings and examples of changing them.

If you need to debug the system further, you can enable the admin container,
which enables SSH access to the system using the key you specified when you
launched the instance.

To enable the admin container, run:

   enable-admin-container

[ssm-user@ip-192-168-97-207 /]$

Here's the helper:

[ssm-user@ip-192-168-97-207 /]$ enable-admin-container
Checking whether there are pending settings; we don't want to silently commit other changes
Setting admin container to enabled
204 No Content
Committing and applying changes
200 OK
["settings.host-containers.admin.enabled"]
The admin container is now enabled - it should pull and start soon, and then you can SSH in

Here's with -x so you can see what it's doing:

sh-4.2$ sh -x /tmp/enable-admin-container 
+ command -v apiclient
+ echo 'Checking whether there are pending settings; we don'\''t want to silently commit other changes'
Checking whether there are pending settings; we don't want to silently commit other changes
++ apiclient -u /settings/pending
+ PENDING='{}'
+ rc=0
+ '[' 0 -ne 0 ']'
+ '[' '{}' '!=' '{}' ']'
+ echo 'Setting admin container to enabled'
Setting admin container to enabled
+ apiclient -v -u /settings -m PATCH -d '{"host-containers": {"admin": {"enabled": true}}}'
204 No Content
+ echo 'Committing and applying changes'
Committing and applying changes
+ apiclient -v -u /settings/commit_and_apply -m POST
200 OK
["settings.host-containers.admin.enabled"]
+ echo 'The admin container is now enabled - it should pull and start soon, and then you can SSH in'
The admin container is now enabled - it should pull and start soon, and then you can SSH in

If there are already pending settings, it doesn't want to quietly commit those, so it bails:

sh-4.2$ sh /tmp/enable-admin-container 
Checking whether there are pending settings; we don't want to silently commit other changes
Error: found pending settings in API, cowardly refusing to commit them.
You can commit them yourself first with `apiclient -u /settings/commit_and_apply -m POST` and try again.
Pending settings: {"host-containers":{"admin":{"enabled":true}}}

You can try manually enabling the admin container like this:
   apiclient -u /settings -m PATCH -d '{"host-containers": {"admin": {"enabled": true}}}'
   apiclient -u /settings/commit_and_apply -m POST

[tjkirch]

This push improves the motd wording per @etungsten.

[zmrow]

Other than the wording issue, I'm happy with this.

Nice work! 🌖