Always bust builder cache building the container image

Do not use by default the builder cache, when we build the Dangerzone
container image. This way, we can always have the most fresh result when
we run the `./install/common/build-image.py` command.

If a dev wants to speed up non-release builds, we add the `--use-cache`
flag to use the builder cache.

install/common/build-image.py

@@ -37,28 +37,36 @@ def main():
         default=9,
         help="The Gzip compression level, from 0 (lowest) to 9 (highest, default)",
     )
+    parser.add_argument(
+        "--use-cache",
+        action="store_true",
+        help="Use the builder's cache to speed up the builds (not suitable for release builds)",
+    )
     args = parser.parse_args()
 
     print(f"Building for architecture '{ARCH}'")
 
     print("Exporting container pip dependencies")
     with ContainerPipDependencies():
-        print("Pulling base image")
-        subprocess.run(
-            [
-                args.runtime,
-                "pull",
-                "alpine:latest",
-            ],
-            check=True,
-        )
+        if not args.use_cache:
+            print("Pulling base image")
+            subprocess.run(
+                [
+                    args.runtime,
+                    "pull",
+                    "alpine:latest",
+                ],
+                check=True,
+            )
 
         print("Building container image")
+        cache_args = [] if args.use_cache else ["--no-cache"]
         subprocess.run(
             [
                 args.runtime,
                 "build",
                 BUILD_CONTEXT,
+                *cache_args,
                 "--build-arg",
                 f"REQUIREMENTS_TXT={REQUIREMENTS_TXT}",
                 "--build-arg",