Add ACMESH_VERSION build arg to the Dockerfile

Dockerfile

@@ -24,6 +24,8 @@ FROM alpine:3.12
 LABEL maintainer="Nicolas Duchon <[email protected]> (@buchdag)"
 
 ARG GIT_DESCRIBE
+ARG ACMESH_VERSION=2.8.7
+
 ENV COMPANION_VERSION=$GIT_DESCRIBE \
     DOCKER_HOST=unix:///var/run/docker.sock \
     PATH=$PATH:/app

install_acme.sh

@@ -5,12 +5,13 @@ set -e
 # Install git (required to fetch acme.sh)
 apk --no-cache --virtual .acmesh-deps add git
 
-# Get acme.sh Let's Encrypt client source
-tag="2.8.7"
+# Get acme.sh ACME client source
 mkdir /src
 git -C /src clone https://github.com/Neilpang/acme.sh.git
 cd /src/acme.sh
-git checkout "$tag"
+if [[ "$ACMESH_VERSION" != "master" ]]; then
+  git -c advice.detachedHead=false checkout "$ACMESH_VERSION"
+fi
 
 # Install acme.sh in /app
 ./acme.sh --install \