Add 'dockerfile' as a Dockerfile file name (semgrep#7819)

Dockerfiles can be called `dockerfile` as well as `Dockerfile`. Support
both.

Closes semgrep#7824

Test plan: see tests

PR checklist:

- [x] Purpose of the code is [evident to future
readers](https://semgrep.dev/docs/contributing/contributing-code/#explaining-code)
- [x] Tests included or PR comment includes a reproducible test plan
- [x] Documentation is up-to-date
- [x] A changelog entry was [added to
changelog.d](https://semgrep.dev/docs/contributing/contributing-code/#adding-a-changelog-entry)
for any user-facing change
- [x] Change has no security implications (otherwise, ping security
team)

If you're unsure about any of this, please see:

- [Contribution
guidelines](https://semgrep.dev/docs/contributing/contributing-code)!
- [One of the more specific guides located
here](https://semgrep.dev/docs/contributing/contributing/)

---------

Co-authored-by: Emma Jin <--get>
Co-authored-by: github-actions <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Martin Jambon <[email protected]>

changelog.d/gh-7824.fixed

@@ -0,0 +1 @@
+When scanning with `-l dockerfile`, files named `dockerfile` as well as `Dockerfile` will be scanned.

cli/tests/e2e/snapshots/test_spacegrep/test_spacegrep/rulesspacegrepdockerfile.yaml-spacegrepdockerfile/results.json

@@ -0,0 +1,88 @@
+{
+  "errors": [],
+  "paths": {
+    "_comment": "<add --verbose for a list of skipped paths>",
+    "scanned": [
+      "targets/spacegrep/dockerfile"
+    ]
+  },
+  "results": [
+    {
+      "check_id": "rules.spacegrep.double-root",
+      "end": {
+        "col": 10,
+        "line": 3,
+        "offset": 43
+      },
+      "extra": {
+        "engine_kind": "OSS",
+        "fingerprint": "0x42",
+        "is_ignored": false,
+        "lines": "USER root",
+        "message": "'USER' is specified twice\n",
+        "metadata": {},
+        "metavars": {
+          "$ROOT": {
+            "abstract_content": "root",
+            "end": {
+              "col": 10,
+              "line": 3,
+              "offset": 43
+            },
+            "start": {
+              "col": 6,
+              "line": 3,
+              "offset": 39
+            }
+          }
+        },
+        "severity": "ERROR"
+      },
+      "path": "targets/spacegrep/dockerfile",
+      "start": {
+        "col": 1,
+        "line": 3,
+        "offset": 34
+      }
+    },
+    {
+      "check_id": "rules.spacegrep.double-root",
+      "end": {
+        "col": 10,
+        "line": 7,
+        "offset": 111
+      },
+      "extra": {
+        "engine_kind": "OSS",
+        "fingerprint": "0x42",
+        "is_ignored": false,
+        "lines": "USER root",
+        "message": "'USER' is specified twice\n",
+        "metadata": {},
+        "metavars": {
+          "$ROOT": {
+            "abstract_content": "root",
+            "end": {
+              "col": 10,
+              "line": 7,
+              "offset": 111
+            },
+            "start": {
+              "col": 6,
+              "line": 7,
+              "offset": 107
+            }
+          }
+        },
+        "severity": "ERROR"
+      },
+      "path": "targets/spacegrep/dockerfile",
+      "start": {
+        "col": 1,
+        "line": 7,
+        "offset": 102
+      }
+    }
+  ],
+  "version": "0.42"
+}

cli/tests/e2e/targets/spacegrep/dockerfile

@@ -0,0 +1,8 @@
+FROM alpine
+# ruleid: double-root
+USER root
+RUN apk install curl
+CMD ["/hello"]
+# ruleid: double-root
+USER root
+CMD ["ls -ltr"]

cli/tests/e2e/test_spacegrep.py

@@ -11,6 +11,7 @@
         ("rules/spacegrep/markdown.yaml", "spacegrep/markdown.md"),
         ("rules/spacegrep/httpresponse.yaml", "spacegrep/httpresponse.txt"),
         ("rules/spacegrep/dockerfile.yaml", "spacegrep/root.Dockerfile"),
+        ("rules/spacegrep/dockerfile.yaml", "spacegrep/dockerfile"),
         ("rules/spacegrep/multi-lines.yaml", "spacegrep/multi-lines.java"),
     ],
 )