Allow SHA-256 digests in container image references

[lpcox]

Problem

The container field in MCP server configuration rejects @sha256: digest-pinned image references. The current regex pattern only allows tag-based references (image:tag):

// internal/config/validation_schema.go:62
containerPattern = regexp.MustCompile(`^[a-zA-Z0-9][a-zA-Z0-9./_-]*(:([a-zA-Z0-9._-]+|latest))?$`)

// internal/config/schema/mcp-gateway-config.schema.json
"pattern": "^[a-zA-Z0-9][a-zA-Z0-9./_-]*(:([a-zA-Z0-9._-]+|latest))?$"

This means digest-pinned references like ghcr.io/github/github-mcp-server:v0.32.0@sha256:2763823c... are rejected during config validation, preventing immutable container image pinning.

Context

The gh-aw compiler is adding SHA-256 digest pinning for supply-chain security (github/gh-aw#25072). The compiler currently works around this by stripping the @sha256: part before writing the container field and only using the full digest-pinned ref in docker pull commands. Supporting digests natively in the gateway would be cleaner and allow operators to pin images in their own configs.

Proposed Fix

1. Update the regex pattern

Allow an optional @sha256:<hex> suffix after the tag:

// Allows: image, image:tag, image@sha256:abc123, image:tag@sha256:abc123
containerPattern = regexp.MustCompile(`^[a-zA-Z0-9][a-zA-Z0-9./_-]*(:([a-zA-Z0-9._-]+|latest))?(@sha256:[a-fA-F0-9]{64})?$`)

2. Update the JSON schema

{
  "pattern": "^[a-zA-Z0-9][a-zA-Z0-9./_-]*(:([a-zA-Z0-9._-]+|latest))?(@sha256:[a-fA-F0-9]{64})?$"
}

3. Pass through the full reference to Docker

Ensure the launcher passes the complete image:tag@sha256:... reference to docker run so Docker resolves by content address.

4. Add tests

• Valid: ghcr.io/github/github-mcp-server:v0.32.0@sha256:2763823c... (tag + digest)
• Valid: ghcr.io/github/github-mcp-server@sha256:2763823c... (digest only, no tag)
• Valid: ghcr.io/github/github-mcp-server:v0.32.0 (tag only, existing behavior)
• Invalid: ghcr.io/github/github-mcp-server@sha256:short (digest too short)
• Invalid: ghcr.io/github/github-mcp-server@md5:abc (wrong algorithm)

Files to Change

• internal/config/validation_schema.go — containerPattern regex
• internal/config/schema/mcp-gateway-config.schema.json — container property pattern
• internal/config/validation_schema_test.go — add digest test cases
• internal/config/validation_string_patterns_test.go — add digest pattern tests
• internal/config/rules/rules_test.go — add valid digest case

References

• Pin all MCP container images to SHA-256 digests; replace floating tags gh-aw#25072 — compiler PR adding digest pinning
• github/gh-aw#25072 comment — describes the schema rejection and workaround