Support detail view of a tagged image

Running the `dredge <REGISTRY> show <image> <tag>` now responds
with additional output in the form:

```yaml
name: foobar
tag: latest
architecture: amd64
fsLayers:
- blobSum: sha256:a3ed95caeb02ffe68cdd9fd84406680ae93d633cb16422d00e8a7c22955b46d4
- blobSum: sha256:d4228a16bba21ff0eabab548df8f4933103d1a83e5894216c7eb32e3058a8e5e
- blobSum: sha256:1f25ec90921b9d511541f9d38ce2b339de2afffc586e55d75b7345b2057f1993
- blobSum: sha256:a3ed95caeb02ffe68cdd9fd84406680ae93d633cb16422d00e8a7c22955b46d4
- blobSum: sha256:7d97e254a0461b0a30b3f443f1daa0d620a3cc6ff4e2714cc1cfd96ace5b7a7e
digest: sha256:0259571889ac87efbfca5b79a0abe9baf626d058ec5f9a5744bace2229d9ed50
etag: sha256:0259571889ac87efbfca5b79a0abe9baf626d058ec5f9a5744bace2229d9ed50
```

src/commands.rs

@@ -17,6 +17,7 @@
 use std::io::Write;
 
 use serde::Deserialize;
+use serde::Serialize;
 use url::Url;
 
 use crate::api;
@@ -94,16 +95,66 @@ pub async fn tags_handler(
 ///
 /// Returns an `ApiError` if there is a problem fetching the manifest or if there
 /// is a problem parsing the response from the Docker Registry API.
-#[allow(clippy::unused_async)]
+#[allow(clippy::similar_names)]
 pub async fn show_handler(
-    _buf: &mut dyn Write,
+    buf: &mut dyn Write,
     registry_url: &Url,
     image: &str,
     tag: &str,
 ) -> Result<(), ApiError> {
+    #[derive(Debug, Serialize, Deserialize)]
+    #[serde(rename_all = "camelCase")]
+    struct FsLayer {
+        blob_sum: String,
+    }
+
+    #[derive(Debug, Serialize, Deserialize)]
+    struct Response {
+        name: String,
+        tag: String,
+        architecture: String,
+
+        #[serde(rename = "fsLayers")]
+        fslayers: Vec<FsLayer>,
+
+        #[serde(skip_deserializing)]
+        digest: String,
+
+        #[serde(skip_deserializing)]
+        etag: String,
+    }
     log::trace!("show_handler(registry_url: {registry_url:?}, image: {image}, tag: {tag})");
     let path = format!("/v2/{image}/manifests/{tag}");
-    let _url = registry_url.join(&path)?;
+    let url = registry_url.join(&path)?;
+
+    let resp = reqwest::get(url).await?;
+    let headers = resp.headers();
+    let digest: String = String::from(
+        headers
+            .get("docker-content-digest")
+            .ok_or(ApiError::UnexpectedResponse(String::from(
+                "Missing docker-content-digest header",
+            )))?
+            .to_str()?,
+    );
+
+    let etag: String = String::from(
+        headers
+            .get("etag")
+            .ok_or(ApiError::UnexpectedResponse(String::from(
+                "Missing etag header",
+            )))?
+            .to_str()?
+            .strip_prefix("'\"")
+            .and_then(|s| s.strip_suffix("\"'"))
+            .unwrap_or(&digest),
+    );
+
+    let mut body: Response = resp.json().await?;
+    body.digest = digest;
+    body.etag = etag;
+
+    serde_yaml::to_writer(buf, &body)?;
     Ok(())
 }
 
@@ -149,6 +200,7 @@ pub async fn check_handler(buf: &mut dyn Write, registry_url: &Url) -> Result<()
 mod tests {
     use std::error::Error;
 
+    use indoc::indoc;
     use url::Url;
 
     use crate::error;
@@ -383,4 +435,91 @@ mod tests {
         mock_response.assert();
         Ok(())
     }
+
+    /// Validate the happy path for the show handler.
+    ///
+    /// This test spins up a mock server, and makes a request to the image
+    /// manifests endpoint.  It checks that the handler both called the request
+    /// the expected number of times, and did not return an error.
+    #[async_std::test]
+    async fn test_show_handler() {
+        let mut server = mockito::Server::new_async().await;
+        let path = "/v2/foo/manifests/latest";
+
+        let response_body = r#"
+        {
+               "schemaVersion": 1,
+               "name": "foo",
+               "tag": "latest",
+               "architecture": "amd64",
+               "fsLayers": [
+                  {
+                     "blobSum": "sha256:a3ed95caeb02ffe68cdd9fd84406680ae93d633cb16422d00e8a7c22955b46d4"
+                  },
+                  {
+                     "blobSum": "sha256:7d97e254a0461b0a30b3f443f1daa0d620a3cc6ff4e2714cc1cfd96ace5b7a7e"
+                  }
+               ],
+               "history": [
+                  {
+                     "v1Compatibility": "{\"id\":\"7fe38ce3fe63caeaacf6be64933d0d55adc5c5f48762b20ec6129d1a41691a84\",\"parent\":\"8ca907037d044ff942e9c95562b786f1913d3b05a4bda16ad3ed3e7ee67e8c76\",\"created\":\"2023-09-07T00:21:13.838729514Z\",\"container_config\":{\"Cmd\":[\"/bin/sh -c #(nop)  CMD [\\\"bash\\\"]\"]},\"throwaway\":true}"
+                  },
+                  {
+                     "v1Compatibility": "{\"id\":\"8ca907037d044ff942e9c95562b786f1913d3b05a4bda16ad3ed3e7ee67e8c76\",\"created\":\"2023-09-07T00:21:13.444807009Z\",\"container_config\":{\"Cmd\":[\"/bin/sh -c #(nop) ADD file:cb5fcc80c057b356a31492a20c6e3a75b70ed70a663506c8e97ad730ae32a02d in / \"]}}"
+                  }
+               ],
+               "signatures": [
+                  {
+                     "header": {
+                        "jwk": {
+                           "crv": "P-256",
+                           "kid": "7ZLW:DJCO:GYG4:DCZD:TRO6:QW3Y:Q7Q3:PTXB:JDQX:4DLY:NB2B:4GJJ",
+                           "kty": "EC",
+                           "x": "LXquBoF1_XI3fawa-7UW9Y1Le7j7FiDGS3KB_4gF5hY",
+                           "y": "UT5SniKpELMqL-j9YwL2fZLUHmRIFwori9rUBG18b_k"
+                        },
+                        "alg": "ES256"
+                     },
+                     "signature": "5_paRRhUCmwkAZJrjBfbvOJ341atEjUQuhG7i4kITyG3e_U2yuDqs9X7bHHMtmUTbChSp59NHi124uauAjoxIg",
+                     "protected": "eyJmb3JtYXRMZW5ndGgiOjI3MDIsImZvcm1hdFRhaWwiOiJDbjAiLCJ0aW1lIjoiMjAyMy0wOS0yN1QxMzoyMTo1MloifQ"
+                  }
+               ]
+            }
+        "#;
+        // Mock the HTTP response for the Docker Registry API
+        let registry_url = Url::parse(&server.url()).expect("Failed to parse registry URL");
+        let mock_response = server
+            .mock("GET", path)
+            .with_status(http::status::StatusCode::OK.as_u16().into())
+            .with_header(http::header::CONTENT_TYPE.as_str(), "application/json")
+            .with_header(
+                "docker-content-digest",
+                "sha256:0259571889ac87efbfca5b79a0abe9baf626d058ec5f9a5744bace2229d9ed50",
+            )
+            .with_header(
+                "etag",
+                "sha256:0259571889ac87efbfca5b79a0abe9baf626d058ec5f9a5744bace2229d9ed50",
+            )
+            .with_body(response_body)
+            .create();
+
+        let mut buf: Vec<u8> = Vec::new();
+        let result = show_handler(&mut buf, &registry_url, "foo", "latest").await;
+
+        let expected_body = indoc! {"
+        name: foo
+        tag: latest
+        architecture: amd64
+        fsLayers:
+        - blobSum: sha256:a3ed95caeb02ffe68cdd9fd84406680ae93d633cb16422d00e8a7c22955b46d4
+        - blobSum: sha256:7d97e254a0461b0a30b3f443f1daa0d620a3cc6ff4e2714cc1cfd96ace5b7a7e
+        digest: sha256:0259571889ac87efbfca5b79a0abe9baf626d058ec5f9a5744bace2229d9ed50
+        etag: sha256:0259571889ac87efbfca5b79a0abe9baf626d058ec5f9a5744bace2229d9ed50\n"
+        };
+
+        assert!(result.is_ok());
+        assert_eq!(String::from_utf8(buf).unwrap(), *expected_body);
+
+        mock_response.assert();
+    }
 }