Make image build logs verbose if necessary

The `-q` flag is not implemented by `docker buildx`, which results in an
output:

```
WARN[0000] quiet currently not implemented.
```

In the same way, the build output is not logged to `stdout` (but
`stderr`). This means we now dump the whole build process to a file and
if the `docker buildx` command fails, then we output those logs by
printing the contents of that file. This will also reduce the overall
verbosity and aligns to the original `docker build` behavior.

Signed-off-by: Sascha Grunert <mail@saschagrunert.de>

build/lib/release.sh

@@ -383,12 +383,16 @@ EOF
           echo "COPY nsswitch.conf /etc/" >> "${docker_file_path}"
         fi
 
-        DOCKER_CLI_EXPERIMENTAL=enabled "${DOCKER[@]}" buildx build \
+        local build_log="${docker_build_path}/build.log"
+        if ! DOCKER_CLI_EXPERIMENTAL=enabled "${DOCKER[@]}" buildx build \
           --platform linux/"${arch}" \
           --load ${docker_build_opts:+"${docker_build_opts}"} \
-          -q \
           -t "${docker_image_tag}" \
-          "${docker_build_path}" >/dev/null
+          "${docker_build_path}" >"${build_log}" 2>&1; then
+            cat "${build_log}"
+            exit 1
+        fi
+        rm "${build_log}"
 
         # If we are building an official/alpha/beta release we want to keep
         # docker images and tag them appropriately.