static-linux: allow setting TREE to build from arbitrary git commit.

static-linux/Dockerfile

@@ -20,6 +20,7 @@ RUN stack install --stack-yaml stack.pkg.yaml --only-dependencies \
     --test --ghc-options '-O2 -optc-Os -optl-static -fPIC' \
     pandoc pandoc-citeproc
 CMD git pull && \
+    git checkout -b work $TREE && \
     export VERSION=`grep '^Version:' pandoc.cabal | awk '{print $$2;}'` && \
     stack install --stack-yaml stack.pkg.yaml \
       --local-bin-path /artifacts --flag 'pandoc:embed_data_files' \

static-linux/Makefile

@@ -1,11 +1,12 @@
+TREE?=HEAD
 ARTIFACTS=`pwd`/artifacts
 
 build:
 	mkdir -p $(ARTIFACTS)
-	docker run -v $(ARTIFACTS):/artifacts alpine-pandoc
+	docker build -t alpine-pandoc .
+	docker run --env TREE=$(TREE) -v $(ARTIFACTS):/artifacts alpine-pandoc
 
 setup:
 	docker pull alpine:edge
-	docker build -t alpine-pandoc .
 
 .PHONY: build setup