Add ffmpeg to Docker image

Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1-alpine AS builder
+FROM golang:1-alpine3.12 AS builder
 
 RUN echo "@edge_community http://dl-cdn.alpinelinux.org/alpine/edge/community" >> /etc/apk/repositories
 RUN apk add --no-cache git ca-certificates build-base su-exec olm-dev@edge_community
@@ -7,13 +7,13 @@ COPY . /build
 WORKDIR /build
 RUN go build -o /usr/bin/mautrix-whatsapp
 
-FROM alpine:latest
+FROM alpine:3.12
 
 ENV UID=1337 \
     GID=1337
 
 RUN echo "@edge_community http://dl-cdn.alpinelinux.org/alpine/edge/community" >> /etc/apk/repositories
-RUN apk add --no-cache su-exec ca-certificates olm@edge_community
+RUN apk add --no-cache ffmpeg su-exec ca-certificates olm@edge_community
 
 COPY --from=builder /usr/bin/mautrix-whatsapp /usr/bin/mautrix-whatsapp
 COPY --from=builder /build/example-config.yaml /opt/mautrix-whatsapp/example-config.yaml

Dockerfile.ci

@@ -1,10 +1,10 @@
-FROM alpine:latest
+FROM alpine:3.12
 
 ENV UID=1337 \
     GID=1337
 
 RUN echo "@edge_community http://dl-cdn.alpinelinux.org/alpine/edge/community" >> /etc/apk/repositories
-RUN apk add --no-cache su-exec ca-certificates olm@edge_community
+RUN apk add --no-cache ffmpeg su-exec ca-certificates olm@edge_community
 
 ARG EXECUTABLE=./mautrix-whatsapp
 COPY $EXECUTABLE /usr/bin/mautrix-whatsapp