chore: add slackv3 build to Dockerfile

Dockerfile

@@ -1,17 +1,22 @@
-FROM python:3.9-slim as BUILD
+FROM python:3.9-slim as build
 WORKDIR /wheel
 COPY . .
-RUN apt update && apt install build-essential -y
+RUN apt update && apt install -y build-essential git
+RUN cd /tmp && \
+    git clone https://github.com/errbotio/err-backend-slackv3 slackv3
 RUN pip3 wheel --wheel-dir=/wheel . \
-    errbot errbot[irc] errbot[slack] errbot[XMPP] errbot[telegram]
+    -r /tmp/slackv3/requirements.txt wheel \
+    errbot errbot[irc] errbot[XMPP] errbot[telegram] && \
+    cp /tmp/slackv3/requirements.txt /wheel/slackv3-requirements.txt
 
 FROM python:3.9-slim
-COPY --from=BUILD /wheel /wheel
+COPY --from=build /wheel /wheel
 RUN apt update && \
     apt install -y git && \
     cd /wheel && \
     pip3 -vv install --no-cache-dir --no-index --find-links /wheel . \
-    errbot errbot[irc] errbot[slack] errbot[XMPP] errbot[telegram] && \
+    -r /wheel/slackv3-requirements.txt \
+    errbot errbot[irc] errbot[XMPP] errbot[telegram] && \
     rm -rf /wheel /var/lib/apt/lists/*
 
 RUN useradd -m errbot