Update error hint and schema description to mention @sha256 digest support

Agent-Logs-Url: https://github.com/github/gh-aw-mcpg/sessions/c0572733-6f1d-4e16-98d8-4d1cafd9413b

Co-authored-by: lpcox <15877973+lpcox@users.noreply.github.com>

Author: Copilot

internal/config/schema/mcp-gateway-config.schema.json

@@ -74,7 +74,7 @@
         },
         "container": {
           "type": "string",
-          "description": "Container image for the MCP server (e.g., 'ghcr.io/example/mcp-server:latest'). This field is required for stdio servers per MCP Gateway Specification section 4.1.2.",
+          "description": "Container image for the MCP server (e.g., 'ghcr.io/example/mcp-server:latest' or 'ghcr.io/example/mcp-server@sha256:<digest>'). This field is required for stdio servers per MCP Gateway Specification section 4.1.2.",
           "minLength": 1,
           "pattern": "^[a-zA-Z0-9][a-zA-Z0-9./_-]*(:([a-zA-Z0-9._-]+|latest))?(@sha256:[a-fA-F0-9]{64})?$"
         },

internal/config/validation_schema.go

@@ -506,7 +506,7 @@ func validateStringPatterns(stdinCfg *StdinConfig) error {
 			if server.Container != "" && !containerPattern.MatchString(server.Container) {
 				return rules.InvalidPattern("container", server.Container,
 					fmt.Sprintf("%s.container", jsonPath),
-					"Use a valid container image format (e.g., 'ghcr.io/owner/image:tag' or 'owner/image:latest')")
+					"Use a valid container image format (e.g., 'ghcr.io/owner/image:tag', 'owner/image:latest', or 'ghcr.io/owner/image:tag@sha256:<digest>')")
 			}
 
 			// Validate mount patterns