Take the image signing key from PIXA_SIGNING_KEY and refuse the example placeholder (closes #110)

The Docker image now ships config.docker.yml, which sets only signing_key (read from the PIXA_SIGNING_KEY environment variable), state_dir and port. The placeholder key and the five-host allowlist from config.example.yml are no longer in the image; anything else is configured by mounting a file over /etc/pixa/config.yml. A container started without PIXA_SIGNING_KEY exits naming it.

Startup now refuses the exact placeholder signing_key from config.example.yml. It is 45 characters long and used to pass the length check, so a deployment could sign URLs with a key that is public in this repository. README Getting Started is corrected to match.

What a reader would trip over: the unset-variable error comes from config interpolation, not from validate(); the signing key checks moved into validateSigningKey to stay under the complexity limit.

Disclosure: TODO.md is not updated by this change.

Model: opus-4-8 (implementation, review); fable-5-1 (landing message)

internal/config/config_validation_internal_test.go

@@ -303,6 +303,11 @@ func invalidHostAndCredentialCases() []abortCase {
 			yaml:              "signing_key: short\n",
 			wantErrSubstrings: []string{keySigningKey},
 		},
+		{
+			name:              "signing_key is the documented placeholder",
+			yaml:              "signing_key: " + placeholderSigningKey + "\n",
+			wantErrSubstrings: []string{keySigningKey},
+		},
 		{
 			name:              "signing_key missing",
 			yaml:              "port: 8080\n",