Fix current shell on root when installing svgo (#1644)

This fix the following errors:
/bin/sh: 13: /root/.bashrc: shopt: not found
/bin/sh: 21: /root/.bashrc: shopt: not found
/bin/sh: 103: /root/.bashrc: source: not found

workspace/Dockerfile

@@ -677,7 +677,7 @@ ARG INSTALL_IMAGE_OPTIMIZERS=false
 RUN if [ ${INSTALL_IMAGE_OPTIMIZERS} = true ]; then \
     apt-get install -y jpegoptim optipng pngquant gifsicle && \
     if [ ${INSTALL_NODE} = true ]; then \
-        . ~/.bashrc && npm install -g svgo \
+        exec bash && . ~/.bashrc && npm install -g svgo \
     ;fi\
 ;fi