chore: add the --ephemeral server flag (#16126)

Another PR to address https://github.com/coder/coder/issues/15109.

Changes:
- Introduces the `--ephemeral` flag, which changes the Coder config
directory to a temporary location. The config directory is where the
built-in PostgreSQL stores its data, so using a new one results in a
deployment with a fresh state.

The `--ephemeral` flag is set to replace the `--in-memory` flag once the
in-memory database is removed.

codersdk/deployment.go

@@ -350,6 +350,7 @@ type DeploymentValues struct {
 	ProxyTrustedOrigins             serpent.StringArray                  `json:"proxy_trusted_origins,omitempty" typescript:",notnull"`
 	CacheDir                        serpent.String                       `json:"cache_directory,omitempty" typescript:",notnull"`
 	InMemoryDatabase                serpent.Bool                         `json:"in_memory_database,omitempty" typescript:",notnull"`
+	EphemeralDeployment             serpent.Bool                         `json:"ephemeral_deployment,omitempty" typescript:",notnull"`
 	PostgresURL                     serpent.String                       `json:"pg_connection_url,omitempty" typescript:",notnull"`
 	PostgresAuth                    string                               `json:"pg_auth,omitempty" typescript:",notnull"`
 	OAuth2                          OAuth2Config                         `json:"oauth2,omitempty" typescript:",notnull"`
@@ -2282,6 +2283,15 @@ func (c *DeploymentValues) Options() serpent.OptionSet {
 			Value:       &c.InMemoryDatabase,
 			YAML:        "inMemoryDatabase",
 		},
+		{
+			Name:        "Ephemeral Deployment",
+			Description: "Controls whether Coder data, including built-in Postgres, will be stored in a temporary directory and deleted when the server is stopped.",
+			Flag:        "ephemeral",
+			Env:         "CODER_EPHEMERAL",
+			Hidden:      true,
+			Value:       &c.EphemeralDeployment,
+			YAML:        "ephemeralDeployment",
+		},
 		{
 			Name:        "Postgres Connection URL",
 			Description: "URL of a PostgreSQL database. If empty, PostgreSQL binaries will be downloaded from Maven (https://repo1.maven.org/maven2) and store all data in the config root. Access the built-in database with \"coder server postgres-builtin-url\". Note that any special characters in the URL must be URL-encoded.",