Fix mimir-mixin-tools/screenshots/Dockerfile

[pracucci]

What this PR does:
Given /app is a directory it should end with / according to Dockerfile reference doc. I don't know why it was working to me, but Mauro report it doesn't to him.

Which issue(s) this PR fixes:
N/A

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[replay]

Nice, this fixes the build for me, thanks!

main

replay@mauro-workstation:~/go/src/github.com/grafana/mimir/operations/mimir-mixin-tools/screenshots$ docker build .
Sending build context to Docker daemon  50.18kB
Step 1/6 : FROM node:17-bullseye
 ---> e6bed6a65a54
Step 2/6 : RUN apt-get update && apt-get install -y     libnss3     libatk1.0-0     libatk-bridge2.0-0     libcups2     libdrm2     libxkbcommon-x11-0     libxcomposite-dev     libxdamage1     libxrandr2     libgbm-dev     libasound2     pngquant
 ---> Using cache
 ---> 9894f05e01ea
Step 3/6 : RUN mkdir /app
 ---> Using cache
 ---> 9b544308fbec
Step 4/6 : COPY package*.json app.js /app
When using COPY with more than one source file, the destination must be a directory and end with a /

This branch

replay@mauro-workstation:~/go/src/github.com/grafana/mimir/operations/mimir-mixin-tools/screenshots$ docker build .
Sending build context to Docker daemon  50.18kB
Step 1/6 : FROM node:17-bullseye
 ---> e6bed6a65a54
Step 2/6 : RUN apt-get update && apt-get install -y     libnss3     libatk1.0-0     libatk-bridge2.0-0     libcups2     libdrm2     libxkbcommon-x11-0     libxcomposite-dev     libxdamage1     libxrandr2     libgbm-dev     libasound2     pngquant
 ---> Using cache
 ---> 9894f05e01ea
Step 3/6 : RUN mkdir /app
 ---> Using cache
 ---> 9b544308fbec
Step 4/6 : COPY package*.json app.js /app/
 ---> Using cache
 ---> ad7692fa3598
Step 5/6 : RUN cd /app && npm ci
 ---> Using cache
 ---> f7a3d249b899
Step 6/6 : WORKDIR /app
 ---> Using cache
 ---> 30add787cc31
Successfully built 30add787cc31