Fix dockerfile

Dockerfile.trunk

@@ -25,10 +25,7 @@ ARG QUASSEL_VERSION="0.13.1"
 # setup repo
 RUN mkdir /quassel && \
     git clone -b master --single-branch https://github.com/quassel/quassel /quassel/src && \
-    cd /quassel/src && \
-    if [ ! -z "$QUASSEL_VERSION" ]; then \
-      git checkout $QUASSEL_VERSION; \
-    fi
+    cd /quassel/src
 
 # generate build files
 RUN mkdir /quassel/build && \

Dockerfile.v0.13

@@ -27,9 +27,7 @@ ARG QUASSEL_VERSION="0.13.1"
 RUN mkdir /quassel && \
     git clone -b 0.13 --single-branch https://github.com/quassel/quassel /quassel/src && \
     cd /quassel/src && \
-    if [ ! -z "$QUASSEL_VERSION" ]; then \
-      git checkout $QUASSEL_VERSION; \
-    fi
+    git checkout $QUASSEL_VERSION
 
 # generate build files
 RUN mkdir /quassel/build && \