feat: set SSL_CERT_FILE and provide a Cmd

Two minor "quality of life" improvements:
- automatically set SSL_CERT_FILE environment variable,
  so that programs relying on OpenSSL for certificate
  validation can actually validate certificates
  (the certificates are included no matter what since
  we add the "cacert" package to all iamges)
- if the requested image includes an interactive shell
  (e.g. if it includes the "shell" metapackage), set
  the image Cmd to "bash", which allows to execute
  "docker run nixery.dev/shell" and get a shell)

I'm happy to split this PR in two if you'd like, but
since both features touch the Config structure and are
rather small, I thought it would make sense to bundle
them together.

builder/builder.go

@@ -493,7 +493,15 @@ func BuildImage(ctx context.Context, s *State, image *Image) (*BuildResult, erro
 		return nil, err
 	}
 
-	m, c := manifest.Manifest(image.Arch.imageArch, layers)
+	// If the requested packages include a shell,
+	// set cmd accordingly.
+	cmd := ""
+	for _, pkg := range image.Packages {
+		if pkg == "bashInteractive" {
+			cmd = "bash"
+		}
+	}
+	m, c := manifest.Manifest(image.Arch.imageArch, layers, cmd)
 
 	lw := func(w io.Writer) error {
 		r := bytes.NewReader(c.Config)

manifest/manifest.go

@@ -64,9 +64,10 @@ type imageConfig struct {
 		DiffIDs []string `json:"diff_ids"`
 	} `json:"rootfs"`
 
-	// sic! empty struct (rather than `null`) is required by the
-	// image metadata deserialiser in Kubernetes
-	Config struct{} `json:"config"`
+	Config struct {
+		Cmd []string `json:"cmd,omitempty"`
+		Env []string `json:"env,omitempty"`
+	} `json:"config"`
 }
 
 // ConfigLayer represents the configuration layer to be included in
@@ -83,12 +84,16 @@ type ConfigLayer struct {
 // Outside of this module the image configuration is treated as an
 // opaque blob and it is thus returned as an already serialised byte
 // array and its SHA256-hash.
-func configLayer(arch string, hashes []string) ConfigLayer {
+func configLayer(arch string, hashes []string, cmd string) ConfigLayer {
 	c := imageConfig{}
 	c.Architecture = arch
 	c.OS = os
 	c.RootFS.FSType = fsType
 	c.RootFS.DiffIDs = hashes
+	if cmd != "" {
+		c.Config.Cmd = []string{cmd}
+	}
+	c.Config.Env = []string{"SSL_CERT_FILE=/etc/ssl/certs/ca-bundle.crt"}
 
 	j, _ := json.Marshal(c)
 
@@ -103,7 +108,7 @@ func configLayer(arch string, hashes []string) ConfigLayer {
 // layer.
 //
 // Callers do not need to set the media type for the layer entries.
-func Manifest(arch string, layers []Entry) (json.RawMessage, ConfigLayer) {
+func Manifest(arch string, layers []Entry, cmd string) (json.RawMessage, ConfigLayer) {
 	// Sort layers by their merge rating, from highest to lowest.
 	// This makes it likely for a contiguous chain of shared image
 	// layers to appear at the beginning of a layer.
@@ -122,7 +127,7 @@ func Manifest(arch string, layers []Entry) (json.RawMessage, ConfigLayer) {
 		layers[i] = l
 	}
 
-	c := configLayer(arch, hashes)
+	c := configLayer(arch, hashes, cmd)
 
 	m := manifest{
 		SchemaVersion: schemaVersion,