feat(agent): wire up agentssh server to allow exec into container (#16638)

Builds on top of https://github.com/coder/coder/pull/16623/ and wires up
the ReconnectingPTY server. This does nothing to wire up the web
terminal yet but the added test demonstrates the functionality working.

Other changes:
* Refactors and moves the `SystemEnvInfo` interface to the
`agent/usershell` package to address follow-up from
https://github.com/coder/coder/pull/16623#discussion_r1967580249
* Marks `usershellinfo.Get` as deprecated. Consumers should use the
`EnvInfoer` interface instead.

---------

Co-authored-by: Mathias Fredriksson <mafredri@gmail.com>
Co-authored-by: Danny Kopping <danny@coder.com>

agent/agentcontainers/containers_dockercli.go

@@ -6,7 +6,6 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
-	"os"
 	"os/user"
 	"slices"
 	"sort"
@@ -15,6 +14,7 @@ import (
 	"time"
 
 	"github.com/coder/coder/v2/agent/agentexec"
+	"github.com/coder/coder/v2/agent/usershell"
 	"github.com/coder/coder/v2/codersdk"
 
 	"golang.org/x/exp/maps"
@@ -37,6 +37,7 @@ func NewDocker(execer agentexec.Execer) Lister {
 // DockerEnvInfoer is an implementation of agentssh.EnvInfoer that returns
 // information about a container.
 type DockerEnvInfoer struct {
+	usershell.SystemEnvInfo
 	container string
 	user      *user.User
 	userShell string
@@ -122,26 +123,13 @@ func EnvInfo(ctx context.Context, execer agentexec.Execer, container, containerU
 	return &dei, nil
 }
 
-func (dei *DockerEnvInfoer) CurrentUser() (*user.User, error) {
+func (dei *DockerEnvInfoer) User() (*user.User, error) {
 	// Clone the user so that the caller can't modify it
 	u := *dei.user
 	return &u, nil
 }
 
-func (*DockerEnvInfoer) Environ() []string {
-	// Return a clone of the environment so that the caller can't modify it
-	return os.Environ()
-}
-
-func (*DockerEnvInfoer) UserHomeDir() (string, error) {
-	// We default the working directory of the command to the user's home
-	// directory. Since this came from inside the container, we cannot guarantee
-	// that this exists on the host. Return the "real" home directory of the user
-	// instead.
-	return os.UserHomeDir()
-}
-
-func (dei *DockerEnvInfoer) UserShell(string) (string, error) {
+func (dei *DockerEnvInfoer) Shell(string) (string, error) {
 	return dei.userShell, nil
 }