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docker: dockerfile tips.md

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tags: devops, docker, dockerfile

docker: dockerfile tips

COPY vs ADD

ADD has more features than COPY but it is suggested to use COPY since you seldomly use extra features provided by ADD and the feature may give you unexpected result.

ADD supports adds file from remote URL and extracts a directory from a tarball.

FROM debian:10
ADD  pgbouncer-1.14.0.tar.gz /app/pgbouncer
RUN find /app

Above Dockerfile will extract the tar.gz package and place it under /app/pgbouncer so that you'll get file like /app/pgbouncer/pgbouncer-1.14.0/src/xxx.

If you simply replace all COPY with ADD, you won't get problem at most time but you may extract tarballs unexpectedly.

So COPY is suggested and use only ADD when you need its special features.

COPY

Some tips about COPY:

  • You don't need to RUN mkdir xxx for the COPY target, you can COPY . /a/b/c/d/e and docker will create directories for you.

  • directory and slash. You do need to add slash for command like COPY dir1 /app. It will copy all content in dir1 to /app, you won't get path like /app/dir1. It is same as COPY dir1 /app/ and COPY dir1/ /app/.

If you want /app/dir1, you need COPY dir1 /app/dir1.

  • slash matters when copy single file like COPY nginx.conf /etc/nginx/. You MUST add / suffix to indicate that target is a directory, otherwise it will treat target as a regular file if it is not existed.

  • You CAN NOT copy multiple directories to target directory. COPY dir1 dir2 /app/ works but it actually something like copy dir1/* dir2/* /app/ and you won't get path like /app/dir1/xxx and /app/dir2/xxx. You have to use two COPY commands.

      COPY dir1 /app/dir1
  COPY dir2 /app/dir2

And you'll get error for COPY dir1 dir2 /app since you need to use /app/ to specify that the target is a directory but not a plain file.

  • chown supports. From docker 17.09, you can use COPY --chown=user:group to change owner of added files. It saves an extra RUN chown.

docker buildkit features

The docker buildkit is integrated to docker since 18.06. You can enable it via DOCKER_BUILDKIT=1 docker build.

And for dockerfile, you need to add #syntax=docker/dockerfile:1.2 at beginning to enable it.

mount cache

Sometimes, you may want to share some directories across multiple building, for example, yarn cache directory, maven cache, go build cache, pip download cache. The dockerfile doesn't support feature like docker run -v host_volume:container_volume. However, the buildkit supports RUN --mount=type=cache,target=/xxxx sytax so that /xxxx directory will be cached and reused across buildings. For example:

RUN --mount=type=cache,target=/root/.m2 mvn clean package

By this way, the downloaded packages will be cached in /root/.m2 volume. This volume is shared even though you changed dockerfile name or target image name.

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