Allow setting a tag for the container image

Allow setting a tag for the container image, when building it with the
`build-image.py` script. This should be used for development purposes
only, since the proper image name should be dictated by the script.

install/common/build-image.py

@@ -27,6 +27,29 @@ def str2bool(v):
         raise argparse.ArgumentTypeError("Boolean value expected.")
 
 
+def determine_git_tag():
+    # Designate a unique tag for this image, depending on the Git commit it was created
+    # from:
+    # 1. If created from a Git tag (e.g., 0.8.0), the image tag will be `0.8.0`.
+    # 2. If created from a commit, it will be something like `0.8.0-31-g6bdaa7a`.
+    # 3. If the contents of the Git repo are dirty, we will append a unique identifier
+    #    for this run, something like `0.8.0-31-g6bdaa7a-fdcb` or `0.8.0-fdcb`.
+    dirty_ident = secrets.token_hex(2)
+    return (
+        subprocess.check_output(
+            [
+                "git",
+                "describe",
+                "--long",
+                "--first-parent",
+                f"--dirty=-{dirty_ident}",
+            ],
+        )
+        .decode()
+        .strip()[1:]  # remove the "v" prefix of the tag.
+    )
+
+
 def main():
     parser = argparse.ArgumentParser()
     parser.add_argument(
@@ -55,6 +78,11 @@ def main():
         const=True,
         help="Use the builder's cache to speed up the builds",
     )
+    parser.add_argument(
+        "--tag",
+        default=None,
+        help="Provide a custom tag for the image (for development only)",
+    )
     args = parser.parse_args()
 
     tarball_path = Path("share") / "container.tar.gz"
@@ -62,20 +90,7 @@ def main():
 
     print(f"Building for architecture '{ARCH}'")
 
-    # Designate a unique tag for this image, depending on the Git commit it was created
+    tag = args.tag or determine_git_tag()
-    # from:
-    # 1. If created from a Git tag (e.g., 0.8.0), the image tag will be `0.8.0`.
-    # 2. If created from a commit, it will be something like `0.8.0-31-g6bdaa7a`.
-    # 3. If the contents of the Git repo are dirty, we will append a unique identifier
-    #    for this run, something like `0.8.0-31-g6bdaa7a-fdcb` or `0.8.0-fdcb`.
-    dirty_ident = secrets.token_hex(2)
-    tag = (
-        subprocess.check_output(
-            ["git", "describe", "--long", "--first-parent", f"--dirty=-{dirty_ident}"],
-        )
-        .decode()
-        .strip()[1:]  # remove the "v" prefix of the tag.
-    )
     image_name_tagged = IMAGE_NAME + ":" + tag
 
     print(f"Will tag the container image as '{image_name_tagged}'")