I am having only one issue related to getting the SSH daemon to run on when the container starts.

Dou you need it for local use, just to have access to the console or you have to share console access outside?

In case if you need local access, you can use docker exec command to get access inside of your container and run command.

docker exec -it 'container_name' csession 'instance_name' -UUSER

if you have to share access, I think it is also possible to achieve but with the second container and links between.

(& and !) can be used almost everywhere, but not recommended because both operands will be calculated. And sometimes it can cause some unexpected behaviour. Look at my example

USER>set a=1 if a!$i(a) zw a
a=2

even when the first expression already truth second one also calculated and increased the value

USER>set a=1 if a||$i(a) zw a
a=1

In this case, the first expression already truthy, and in OR condition it's enough to make full logical expression truthy as well, and not need to calculate the second expression.

right from the documentation

You can combine multiple Boolean logical expressions by using logical operators. Like all Caché expressions, they are evaluated in strict left-to-right order. There are two types of logical operators: regular logical operators (& and !) and short-circuit logical operators (&& and ||).

When regular logical operators are used to combine logical expressions, Caché evaluates all of the specified expressions, even when the Boolean result is known before all of the expressions have been evaluated. This assures that all expressions are valid.

When short-circuit logical operators are used to combine logical expressions, Caché evaluates only as many expressions as are needed to determine the Boolean result. For example, if there are multiple AND tests, the first expression that returns 0 determines the overall Boolean result. Any logical expressions to the right of this expression are not evaluated. This allows you to avoid unnecessary time-consuming expression evaluations.

Some commands allow you to specify a comma-separated list as an argument value. In this case, Caché handles each listed argument like an independent command statement. Therefore, IF x=7,y=4,z=2 is parsed as IF x=7 THEN IF y=4 THEN IF z=2, which is functionally identical to the short-circuit logical operators statement IF (x=7)&&(y=4)&&(z=2).

You wrong because you missed something important in the documentation. The third parameter is not about string, it is about the list of chars, and the last one the same. So, it will remove everything at the beginning if any of defined characters it will be. And order does not matter.

Write $ZSTRIP("ZZZZTESTSLOCKTEST","<","ETSZ")

return the same

If you have to remove fixed string, you can use $piece or $replace

Write $p("ZZZZTESTSLOCKTEST","ZZZZTEST",2,*)
SLOCKTEST

Each Application Server uses own Global Buffer plus Server Buffer, to fetch data, but the speed of the connection between those servers also matter.

If you don't care where to run so rare queries, you can do it on Database Server. ECP connection only gives a way to get access to the data in databases. And does not help to call any code on Database Server directly. And since %Net.RemoteConnection already deprecated, you can use other ways to call the main server or any other, and SOAP can be one of the ways, but I would prefer JSON Rest API, which in result will transfer fewer data.

To set any response header, you should overwrite OnPreHTTP method in your CSP Page class.

 Class User.Page Extends %CSP.Page
{

ClassMethod OnPage() As %Status
{
  &html<<html>
<head>
</head>
<body>>
  ; To do...
  &html<</body>
</html>>
  Quit $$$OK
}

/// Event handler for <b>PreHTTP</b> event: this is invoked before
/// the HTTP headers for a CSP page have been sent. All changes to the
/// <class>%CSP.Response</class> class, such as adding cookies, HTTP headers,
/// setting the content type etc. must be made from within the OnPreHTTP() method.
/// Also changes to the state of the CSP application such as changing
/// %session.EndSession or %session.AppTimeout must be made within the OnPreHTTP() method.
/// It is prefered that changes to %session.Preserve are also made in the OnPreHTTP() method
/// as this is more efficient, although it is supported in any section of the page.
/// Return <b>0</b> to prevent <method>OnPage</method> from being called.
ClassMethod OnPreHTTP() As %Boolean [ ServerOnly = 1 ]
{
  Do %response.SetHeader("X-MyHeader", "some info")
  quit 1
}

}