feat: ensure related go source and vscode plugins are installed in devcontainer

.devcontainer/Dockerfile

@@ -7,6 +7,12 @@ RUN apt-get install --no-install-recommends -y build-essential file nano && \
   apt-get autoremove --yes && \
   rm -rf /var/lib/apt/lists/*
 
+RUN wget https://dl.google.com/go/go1.16.10.linux-amd64.tar.gz && \
+  tar -xvf go1.16.10.linux-amd64.tar.gz && \
+  mv go /usr/local
+
+RUN GOROOT=/usr/local/go /usr/local/go/bin/go install golang.org/x/tools/gopls@latest 2>&1
+
 ADD https://raw.githubusercontent.com/dokku/dokku/master/tests/dhparam.pem /mnt/dokku/etc/nginx/dhparam.pem
 
 COPY .devcontainer/bin/ /usr/local/bin/
@@ -15,5 +21,6 @@ RUN make ci-dependencies
 
 COPY . .
 
-ENV DOKKU_HOSTNAME=dokku.me
+ENV DOKKU_HOSTNAME=dokku.me GOROOT=/usr/local/go PATH=/usr/local/go/bin:$PATH PLUGIN_MAKE_TARGET=build
+
 LABEL org.label-schema.schema-version=1.0 org.label-schema.vendor=dokku com.dokku.devcontainer=true