build: add deno installation directory to devcontainer path (#40)

Co-authored-by: slack-samples[bot] <169401055+slack-samples[bot]@users.noreply.github.com>

Author: slack-samples[bot]

.devcontainer/Dockerfile

@@ -14,8 +14,7 @@ USER slackdev
 
 # Install the project runtime
 RUN curl -fsSL https://deno.land/install.sh | sh
-RUN export DENO_INSTALL="/home/slackdev/.deno"
-RUN export PATH="$DENO_INSTALL/bin:$PATH"
+ENV PATH="/home/slackdev/.deno/bin:$PATH"
 
 # Set the working directory
 WORKDIR /workspaces