Use Alpine and Clean Up Dockerfile

[decaffeinatedio]

I ran a quick test to deploy Prebid Cache + Redis with Dokku on a random VPS and noticed a number of issues with the Dockerfile. I think this proposed PR will make things faster, cleaner and more secure.

FWIW, this also shrinks the (local) image size from ~129MB to ~29MB.

Major changes:

• Switch from Ubuntu 22 to Alpine 3.23.
• Support ARM
• Pin Go version for more reproducible builds
• Simplify build and strip out unwanted files with .dockerignore

[linux019]

I'm using a simpler dockerfile, also go vendoring is enabled to prevent modules download at the compile stage every time

ARG GOVERSION=1.25.3

FROM golang:${GOVERSION}-alpine3.22 AS builder
WORKDIR /build/
RUN apk add --no-cache pcre-dev bash gcc musl-dev git
COPY ./ ./
ARG TEST="false"
RUN if [ "$TEST" != "false" ]; then ./validate.sh ; fi
RUN go build -mod=vendor -trimpath -ldflags "\
    -X github.com/prebid/prebid-cache/version.Ver=$(git describe --tags) \
    -X github.com/prebid/prebid-cache/version.Rev=$(git rev-parse HEAD)" .

FROM alpine:3.22 AS release
WORKDIR /usr/local/bin/
COPY --from=builder --chmod=544 /build/prebid-cache .
COPY --from=builder --chmod=444 /build/config.yaml .
RUN adduser -D -H prebid_user
USER prebid_user
EXPOSE 2424 2525
ENTRYPOINT ["/usr/local/bin/prebid-cache"]

[decaffeinatedio]

@linux019 I think your Dockerfile is even cleaner (trimpath is a good improvement, too). I can modify the PR to use that -- my main concerns were targeting ARM and not using a huge Ubuntu base image. Let me know!