revert to a single stage build

this has the disadvantage of being a large image (because it's based on
golang:alpine and not alpine, plus the fact that building the project
creates various cache files, though these cache files are the things we
need in order to speed up compiling the project on the main go build
line. most of the compilation time is spent on the dependenent libraries
(i.e. discordgo, crypto, stdlib, etc).

there is not a functional method of compiling dependencies from a bare
go.mod and go.sum file[1], you must have a valid go project in the
directory for go build all to work, at which point the docker layer
cache has been invalidated by `COPY . /app`. the proposed
`go list -export $(go list -m)/...` does not compile all dependencies
either, showcased by checking the size of $GOCACHE before and after
running `go build all`

without doing funky stuff like bind-mounting a volume container into the
build container[2], inflating the image size for faster compiling seems
to be the best tradeoff, as the image will only stay local anyway

[1] golang/go#27719
[2] https://github.com/banzaicloud/docker-golang

Author: alexwh

Dockerfile

@@ -1,18 +1,8 @@
-FROM golang:alpine as dep_builder
-ARG PROJPATH=/go/src/github.com/BEANSQUAD/paul-bot
-RUN apk add --no-cache git gcc libc-dev
-WORKDIR $PROJPATH
-COPY go.mod go.sum ./
+FROM golang:alpine
+RUN apk add --no-cache git gcc libc-dev ca-certificates
+WORKDIR /app
+COPY go.mod go.sum /app
 RUN go mod download
-RUN go build -v all
-
-FROM dep_builder as proj_builder
-ARG PROJPATH=/go/src/github.com/BEANSQUAD/paul-bot
-COPY . $PROJPATH
-COPY --from=dep_builder /root/.cache/go-cache /root/.cache/go-cache
+COPY . /app
 RUN go build -v -a -o paul-bot main.go
-
-FROM alpine
-RUN apk add --no-cache ca-certificates
-COPY --from=proj_builder $PROJPATH/paul-bot /paul-bot
-CMD ["/paul-bot"]
+CMD ["/app/paul-bot"]