Fixed how docker image version is extracted from github ref

docker/build

@@ -7,12 +7,13 @@ DOCKER_IMAGE="shlinkio/shlink"
 
 # If ref is not develop, then this is a tag. Build that docker tag and also "stable"
 if [[ "$GITHUB_REF" != *"develop"* ]]; then
-  TAGS="-t ${DOCKER_IMAGE}:${GITHUB_REF#?}"
+  VERSION=${GITHUB_REF#refs/tags/v}
+  TAGS="-t ${DOCKER_IMAGE}:${VERSION}"
   # Push stable tag only if this is not an alpha or beta tag
   [[ $GITHUB_REF != *"alpha"* && $GITHUB_REF != *"beta"* ]] && TAGS="${TAGS} -t ${DOCKER_IMAGE}:stable"
 
   docker buildx build --push \
-    --build-arg SHLINK_VERSION=${GITHUB_REF#?} \
+    --build-arg SHLINK_VERSION=${VERSION} \
     --platform ${PLATFORMS} \
     ${TAGS} .