docker: Enable the sample hook files to execute without error (#256)

- Include jq package in the Dockerfile for parsing JSON
- Use sh instead of bash which is not available in Alpine Linux

.hooks/post-create

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 
 id="$TUS_ID"
 offset="$TUS_OFFSET"

.hooks/post-finish

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 
 echo "Upload $TUS_ID ($TUS_SIZE bytes) finished"
 cat /dev/stdin | jq .

.hooks/post-receive

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 
 id="$TUS_ID"
 offset="$TUS_OFFSET"

.hooks/post-terminate

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 
 echo "Upload $TUS_ID terminated"
 cat /dev/stdin | jq .

.hooks/pre-create

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 
 filename=$(cat /dev/stdin | jq .MetaData.filename)
 if [ -z "$filename" ]; then

Dockerfile

@@ -22,7 +22,7 @@ FROM alpine:3.8
 
 COPY --from=builder /go/bin/tusd /usr/local/bin/tusd
 
-RUN apk add --no-cache ca-certificates \
+RUN apk add --no-cache ca-certificates jq \
     && addgroup -g 1000 tusd \
     && adduser -u 1000 -G tusd -s /bin/sh -D tusd \
     && mkdir -p /srv/tusd-hooks \