Added support for building image based on tag.

Dockerfile

@@ -8,9 +8,12 @@ RUN apk add --no-cache \
       make \
       musl-dev
 WORKDIR /usr/src
-RUN git clone https://github.com/ProtonMail/proton-bridge.git proton-bridge
-WORKDIR /usr/src/proton-bridge
-RUN make build-nogui
+
+ARG TAG
+RUN git clone https://github.com/ProtonMail/proton-bridge.git proton-bridge \
+    && cd proton-bridge \
+    && if [ -n "$TAG" ]; then git checkout tags/$TAG; fi \
+    && make build-nogui
 
 # Create a container for the ProtonMail Bridge.
 FROM alpine:edge