Pin alpine Dockerfile version

becuase I was getting snapshot failures in
TestScanImage/Alpine_3.10_image_tar_with_3.18_version_file
such as:

```
@@ -35,7 +35,7 @@

          {
            "name": "ca-certificates-cacert",
            "version": "20191127-r2",
-           "commit": "9677580919b73ca6eff94d3d31b9a846b4e40612",
+           "commit": "f24637bad53762e9a2f847dd2e67bb91b1a615c2",
            "ecosystem": "Alpine:v3.18",
            "compareAs": "Alpine"
          },
```

internal/image/fixtures/test-alpine.Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.10
+FROM alpine:3.10sha256:451eee8bedcb2f029756dc3e9d73bab0e7943c1ac55cff3a4861c52a0fdd3e98
 
 # Switch the version to 3.19 to show the advisories published for the latest alpine versions
 COPY "alpine-3.19-alpine-release" "/etc/alpine-release"